I will update this guide with more specific information on the exact numbers and intricacies for bosses & meatheads, but testing them is more difficult due to bosses requiring you to be on live servers and meatheads severely restricting your loadout if you aren't.

The knife is not included for any monster, because I see no reason to bring the knife. Let me know if you have any information I should add.

There's not much to say about grunts. They die easily and frequently come in swarms, which lets certain weapons like the heavy knife cleave several at once. The most interesting thing about them is that they sometimes carry items in their hands. Guns make a noise if they hit you, while torches & lanterns will deal fire damage and explode in a blaze of fire respectively. Be careful killing torch grunts near oil puddles or barrels, as the torch can fall into them and ignite them.

I find the heavy knife to be the best against them, with knuckle knife not far behind. Throwing knives work well enough, but having Assailant is very useful for emergencies where you can't wait for them to turn around, risk an arm shot, or have the brief slowdown for throwing the knife.

Knuckle knife: One heavy bodyshot (18 stamina) or two light headshots (14 stamina).
Heavy knife: One light bodyshot (15 stamina) or one heavy attack for a group (25 stamina)
Dusters: One heavy headshot (10 stamina)
Throwing axe: One throw to the body, or one light attack (40 stamina). If you have Assailant one heavy attack for a group (55 stamina).
Gun: One headshot.
Poison: One bodyshot.
Shotgun: One bodyshot.
Bomb lance: One light for a group (11 stamina).

Throwing knife: One throw to the body, or one heavy attack with Assailant (25 stamina). If you throw a knife into their arm or gut you can kill them with pullout + a light attack (20 stamina). Without Assailant you need a heavy and light attack (45 stamina), which isn't typically what you should be doing.

Hives are able to quickly deplete your health while poisoning you from afar, but they're rather frail. Melee attacks are likely to deal at least a very small amount of damage to you and briefly poison you, although it is possible to avoid the poisoning with proper spacing and aim.

I find it far easier to headshot a hive with a melee attack by approaching them diagonally and to the right. They are slightly more durable than grunts, so certain things require headshots to oneshot them.

I find the throwing knives or axes to be the best for dealing with them. The best melee would again be the heavy knife, with the knuckle knife not far behind.

Knuckle knife: One heavy headshot (18 stamina), or a heavy and light bodyshot (25 stamina).
Heavy knife: One light headshot (15 stamina) or one heavy bodyshot (25 stamina).
Dusters: One heavy headshot (10 stamina).
Throwing axe: One throw to the body, one light attack to the head (40 stamina), or one heavy attack to the body (55 stamina).
Fusees: One bodyshot.
Flare Gun: One bodyshot.
Gun: One headshot.
Shotgun: One bodyshot.
Bomb lance: One light (11 stamina).

Throwing knife: Two thrown bodyshots, one thrown headshot, one thrown bodyshot that you then pull out, or one heavy attack with Assailant (25 stamina). I find bodyshots to be the best option most of the time, although headshots are quite fun. There's a small deviation in exactly how the knife flies if you aren't crouching, which can make headshots much more difficult.

Killing the Hive immediately dissipates the swarm, but that's not always possible. Bees spawned from barrels or hive bombs have no hive to kill, so you have to either wait them out or swat them away.

Knuckle knife: Two light attacks (14 stamina).
Heavy knife: One heavy attack and one light attack (40 stamina).
Dusters: One heavy attack and one light attack (15 stamina).
Throwing axe: Two light attacks (80 stamina). Might require Assailant.
Large gun: Two heavies (50 stamina), or a heavy and two lights.
Small gun: Three heavies (60 stamina).
Fusees, Flare Gun, and I think other sources of fire like lanterns or dragon's breath: One shot.
Flashbomb: One bomb.
Choke bomb: One inhale.

Armored have a very high resistance to bullets, but a somewhat recent patch made them very vulnerable to shotguns instead of very resistant. For some reason they're worth twice the experience of a hive, and slightly more than an immolator. I'm not sure if the melee attacks need to be headshots, but the ranged weapons have correct information.

I find the throwing knives or axes to be the best for them. The best melee is again the heavy knife, with the knuckle knife not far behind. Dusters are horrible.

Knuckle knife: Alternating two heavy attacks and two light attacks (50 stamina).
Heavy knife: Two heavy attacks and one light attack (65 stamina). The light can be done last while exhausted, so only 26 stamina is needed.
Dusters: Four heavy attacks (40 stamina).
Throwing axe: One throw to the body.
Fusees: One bodyshot.
Flare Gun: One bodyshot.
Poison: One bodyshot.
Shotgun: One bodyshot.
Bomb lance: One heavy (50 stamina) or two lights (22 stamina). This one is interesting because the lights are far cheaper, but take more time.

Throwing knife: Two thrown bodyshots + two pullouts. The first pullout will stagger the armored, so you can do both throws & pullouts while right next to him without getting hit.

These guys hurt you when you're near them and hurt you when you melee them, unless you perfectly space an extremely long range attack. They're also more durable than normal, for some reason. If you don't have something to deal with them you should probably just ignore them, and if you can't ignore them I'd just shoot them.

Throwing knife: 3 bodyshots.
Throwing axe: 2 bodyshots or 1 headshot.
Fusees: One bodyshot.
Flare Gun: One bodyshot.
Poison: One bodyshot.
Shotgun: One bodyshot.
Bomb lance: One heavy & one light (61 stamina). You can barely stop yourself from losing a small health chunk if you buffer all 3 inputs.

Hellhounds are fast, janky, frequently come in large packs, cause bleeding, have weird animations, and often wear metal helmets for some reason. Getting caught fighting them is likely a death sentence unless the person who finds you is insanely inaccurate.

I find the most effective option to be the heavy knife, with throwing axes not far behind. Throwing knives are the "best" option, but can be difficult to use.

Knuckle knife: Two light bodyshots (14 stamina).
Heavy knife: One heavy bodyshot that can cleave a group (25 stamina) or one light headshot (15 stamina). I prefer doing bodyshots even for individual hounds, trying to consistently get melee headshots is very hard.
Dusters:
Throwing axes: One light headshot (40 stamina), one heavy bodyshot that can cleave a group (55 stamina), or one thrown bodyshot.
Gun: One headshot.
Poison: One bodyshot.
Shotgun: One bodyshot.
Bomb lance: One light that can cleave a group (11 stamina).

Throwing knives: One thrown headshot, one thrown bodyshot + pullout, or one heavy Assailant bodyshot (25 stamina). I find the most consistent way to take them out is to aggro them and wait for them to turn to face me, then throw the knife at their head while they're preparing to run at me or are currently running at me. If they're already very close a throw + pullout or heavy Assailant is much more feasible than trying to headshot them when they're jumping inside you.

Immolators are the smartest and fastest AI, with a large detection range and the ability to do things like take an alternate path instead of just stare at the door you just closed. If their skin is pierced by most non-blunt attacks they will explode like a firebomb, then become frenzied. While frenzied they will be much more active & aggressive while dealing minor fire damage, but they die after awhile. Poison bullets don't pierce their skin, but poison arrows do for some reason. Clever players can intentionally ignite immolators that are near enemies.

I find the knuckle knife to be the best way to deal with them, but they aren't typically enough of an issue where buttstock bashing is insufficient. Keep in mind that they can't stagger during their stagger animation, so it's much easier to fight them if you time your hits to land as soon as the stagger ends.

Knuckle knife: Four lights (28 stamina).
Dusters: Three heavies (30 stamina).
Large weapon: Four heavies (100 stamina).
Poison: One bodyshot. If you're too far a bodyshot may not be lethal, but he won't ignite.
Choke bomb: One... Bodyshot?

Throwing knife: 5 bodyshots or 3 headshots.

Waterdevils are less of an enemy and more of a map hazard. Water is already very dangerous to cross since you move very slowly and loudly through it, then these guys come in to make it even louder and deal approximately 8.2 billion DPS while bleeding you. If you manage to kill one it respawns in only a few seconds, giving just a brief moment where you can try to pass the water. It's usually much easier to just avoid them rather than fight them.

Poison: One headshot (they only have one hitbox, but since it's not labelled as a head or body, you can call it the head and feel better about yourself).
Guns: Not too many shots.
Bomb lance: One steel ball shot, or one heavy (50 stamina). Very risky to try to jump in and hit them midair, but if they're near the shoreline you can melee them safely.
Throwing axes: Two headshots. Recovering the axes may prove difficult.

Throwing knives: Two headshots. Recovering the knives may prove difficult, but it's only at worst a third of your knives and you get them all back from one resupply.

Meatheads are a very interesting enemy. They're extremely durable and powerful, but in theory they're also deaf and blind so they can only find you based on feeling, the calls of their leeches, or their poison sense if you're poisoned. They work like this in Trials, but for some reason on live servers they don't work and will frequently just aimbot onto you as if you were poisoned. Attacking them, getting too close, or dodging the attack of a leech can all trigger this false poison-sense.

They're one of the most difficult enemies to take down, and uniquely are the only non-boss enemy that can reward you. Upon death there is a 50% chance they drop a random perk. People overestimate how difficult it is to kill them, but with some experience you can consistently take them down rather quickly. Unlike previous monsters I'll break them down into three sections: safe, risky, and dangerous. Safe are things that you can do even with hunters nearby. Risky are things that are safe & easy, but are a very bad idea to try with hunters nearby. Dangerous are things that are dangerous even with no hunters nearby, but can be done with practice.

Keep in mind that the meathead's hitboxes work like everything else. The upper chest takes more damage than the gut, which takes more than the arms & legs. It's a misconception that they take more damage in the leeches coming out of their chest. They technically do, but only because it's part of their upper chest hitbox, and will take that same high damage from the back or side without leeches.

Safe
Any explosion (Red barrel, dynamite stick, etc). Frag bombs aren't explosions.
Bomb lance: One base harpoon.

Throwing knives: Breaking a lantern or killing a torch or lantern grunt who's next to a red barrel that the fire drops onto.

Risky
Guns: It's loud, takes a lot of ammo, and varies in speed drastically depending on your gun, but it's really easy. Depending on your gun you'll probably need 10+ shots to kill them, although poison ammo will probably be around 6-10 shots.
Bomb lance: 3 steel ball shots.
Throwing axes: 4 bodyshots + 2 pullout.

Throwing knives: 9 bodyshots + 3 pullout.

Dangerous
Bomb lance: 6 heavy attacks, or 1 steel ball shot and 3 heavy attacks.
Other melees: I haven't tested them all, but they all work with different speeds. Higher damage per hit ones are obviously better.

Meleeing a meathead seems impossible at first, but with enough practice you can do it without getting hit (as long as a hunter doesn't see you, he'll probably kill you since you're in the worst position imaginable). I'm not exactly sure how to put it into words and will try to figure out exactly how they work so I can describe it better, but the more you fight them the more of a feeling you get on how you should fight them. You need to learn when they do their stupid lock-on thing where they just chase you down hard, and when they aren't doing that you can sort of "riposte" them since they get locked into a swing animation after you attack them. When you hit them go behind them and out of range of their counterattack, then just after the attack ends hit them in the back. I'm sure you can figure out exactly how to do it by practicing.

Not that you need to. The other methods of killing meatheads are sufficient, when you do actually need to kill them instead of just avoid them. But it's somewhat fun to fight them, and it can give you a lot of book of weapons experience.

Bosses are for the most part fought in the same basic way, with minor or major alterations depending on the boss. I will first describe ways of killing a generic boss, then elaborate and mention any other changes in their specific sections.

Guns
Shooting bosses is a very easy way to kill them. It takes ammo and alerts everyone of your position, but banishing is going to alert everyone anyways and if they collected three clues they know where it is anyways. You can always transition into the "shoot him until he dies" strategy when his health gets low, since they're about to know where you are anyways. Just keep in mind that an attentive player will learn what guns you have, and if you shoot him with a shotgun it's very easy for everyone to hear the distinctive shotgun sound. Shotguns (especially with pennyshot), poison ammo, incendiary ammo, the Nitro Express, and rapid weapons (fanning/levering/avtomat) are extremely effective against bosses.

Just remember to attack its weakpoint, for massive damage.

Melee
Bosses are quite vulnerable to melee, but their own attacks are melee so you're putting yourself in range of them when you do it. Depending on the boss you might melee them in different ways, but in general as long as the boss wasn't just attacked you can run up, hit them with a heavy attack, and run away before they retaliate.

Explosives
Explosives do massive damage to bosses, with the big dynamite bundle even being able to oneshot a boss. Explosives can be tricky to use, and improper use could waste the bomb or even get yourself killed, but they're the best way to kill a boss if you have them. Unless you're planning a 250 IQ gigabrain massive prediction hard-read AI manipulating 15-step plan of how to get the boss to walk on the bomb with perfect timing, you'll need to cook the bomb in your hand to throw it at him as it explodes, throw it in front of him then shoot it, or throw it in fire (such as coordinating with a teammate's lantern). Unless cooperating with some fire, other players should stay out of the boss lair, since they can make him move less predictably. You can also just cook the bomb in your hand then sprint at the boss and kamikaze him, since his banishment will restore your lost health. While this is absolutely hilarious, especially if you scream on voice chat since you abruptly cut-out when you die, it is also absolutely throwing if enemy hunters happen to show up and the boss didn't get fully killed by the bomb.

Bomb Lance
The bomb lance's main harpoon attack will 3-shot the bosses, and its heavy melee attack will also do incredible damage. Either and/or both will take any boss out extremely quickly and easily. There isn't much to say, sorry. I just wanted to make sure you know how good the bomb lance is at killing bosses.

Throwing Axes
Combining throwing axes with melee attacks will exponentially speed up the rate at which you can kill them. First, throw all your axes into the boss (weakpoint, preferably). Then, if they're frenzying, wait until it ends before going to the next step. If they're not frenzying run up to them charging a heavy attack, even a tool melee like a knuckle knife will work well with this, and start heavy attacking them while pulling out the axes. The pullout of the axes will stagger them, allowing you to get 2 or 3 heavy attacks with most weapons before needing to duck out. The pullout damage combined with several heavy attacks will do enormous damage.

This strategy also works with throwing knives, but I find the strategy listed there to be better with them.

Throwing Knives
Throwing knives can kill bosses at ludicrous speeds. It's very easy to do. Throw all your knives at their weakpoint, except for one or two. You want to be in the middle of the throw animation of your last knife when they get within range of you to pullout, so that you can continue throwing knives into their weakpoint while pulling them out, which staggers them. You can throw 2-4 knives like this before needing to retreat, but if you try to do it too quickly back-to-back they won't get staggered. If properly done with a good amount of knives you will be able to trigger a frenzy state, removing the need to wait for the stagger cooldown since you're waiting for the frenzy to end.

This strategy also works with throwing axes, but I find the strategy listed there to be better with them.

Resistances: Fire
Weaknesses: Rending, Poison, Explosives, Nitro Express
Weakpoint: None
Frenzy: Dashing, slashing, and leaving a fire trail

The Butcher is a meathead with more lore, and since he's a big meathead that means he has meathead hitboxes. He takes more damage to the upper chest, but has no weakpoint. The "head" is ornamental, and not actually his head. He's practically immune to fire, his attacks will inflict fire, and if he thinks he can't reach you or also sometimes just randomly for no reason he'll throw fireballs.

A very easy boss with nothing to note other than fire doesn't work on him, and if you're struggling against fighting him you can use a choke bomb to extinguish the hook. He can reignite the hook on lanterns, so make sure to turn them off. It's a bit nooby to choke Butcher, but don't let that stop you. If you need to choke him to make it easier, you choke him.

Stamina Shot Showdown
If you have infinite stamina from a stamina shot, Conduit, or maybe even Magpie you can very quickly and easily kill Butcher. Grab an axe, bomb lance, talon weapon, or something else that's strong and just go to town on him. When he frenzies he'll start doing a sprint attack and leaving fire at his feet, but the sprint attack only hits in front of him. If you just stay behind him and keep hitting him over and over again you can kill him in as little as 14 seconds. The fire he drops will burn you, but banishing him will restore your health chunks since fire doesn't do direct damage and only hurts your max health.

Resistances: None
Weaknesses: Piercing, Poison, Fire, Explosives, Nitro Express
Weakpoint: Head
Frenzy: Dashing, slashing, and dropping concertina when damaged

Scrapbeak's backpack provides minor protection against all non-piercing (knuckle knife/pitchfork/saber heavy attack) attacks coming from behind him. He "takes" all items within a large range of his lair (they just don't spawn), and "drops" them with each frenzy phase (he drops better things than what would've actually spawned). He throws concertina bombs if he thinks he can't reach you, or also just randomly for no reason sometimes, and if he takes damage while frenzied he drops concertina at his feet. If explosives or fire are used on him to make him enter a frenzy state, he will drop no items, and if you skip frenzy states with high damage he drops less items. You might want to turn off lanterns in his lair so he doesn't set himself on fire and potentially burn his own drops.

As long as you understand that he drops concertina while frenzied he's a very easy boss. His backpack makes multiple people fighting him slightly less effective, but that's already not too effective to begin with. His drops are SUPER good and they're inside the lair, so it can be a huge boon to forgo fire & explosives for the fight. He has a high chance of dropping perks, frequently drops medkits & toolboxes, and drops tons of DEPLOYED ammo boxes. Like, the type that you bring in as a consumable. The type that restores 20 (special) compact, 10 (special) medium, 5 (special) long, 3 nitro, etc. You basically have infinite ammo, he drops so many of these and they all give so much.

If you want to clear the concertina you can try kiting him into it so he breaks it by walking through it, or you can just clear it yourself. Walk into it and bleed, hit it with sharp things (especially throwing axes, heavy knife, or something bigger like a bomb lance), or throw axes or knives at it. Explosives also clear concertina very well (frags aren't explosives), but he doesn't make very much so a bomb is probably a waste.

Resistances: Explosives
Weaknesses: Piercing, Poison, Fire, Explosives, Nitro Express
Weakpoint: Chest cavity
Frenzy: Clone army

I hate this guy. He sucks. He just hides in bug form for 3 hours because he's too scared to fight me. He knows I'll win. He knows I'm better than him.

Anyways, he's a bit different from Butcher & Scrapbeak. Instead of just running around normally, he's usually in a bug form which is both hard to hit and has an enormous damage resistance that almost makes him invincible in it. He will periodically leave this bug form to either emerge on the ground and do a standard attack which will be interrupted if you hit him, or emerge on the wall to throw a bleeding knife which doesn't seem to be interruptible and also seems to hit you even through walls. When he enters bug form he drops everything embedded in him, which makes throwing knives & axes annoying and stickybombs not usable in their typical way. When he frenzies he sends out a bunch of shadow clones which explode on you to mark you, causing him to focus on trying to attack you. You can just keep sprinting during this and he won't get you, but punching the clones with a light attack will prevent their explosion. You can also dodge it. It seems like if you leave the lair during frenzy he frenzies for way too long.

Because he's so different, some typical strategies might not work very well. Throwing axes/knives will just fall on the floor before you can even start meleeing, so if you're gonna use those just spam pullouts while throwing them at him. Stickybombs have an enormous fuse and fall on the floor, so by yourself it's very difficult to hit him with one. But if you have a teammate, some coordination, a good bit of skill, and a little bit of luck, it's much easier. Have the non-bomber carry a lantern, and coordinate a throw so that the stickybomb lands at his feet and immediately after that the lantern lands on it, causing it to detonate. Both throws should be done at the same time, but if the lantern lands first he'll run away since he hates fire. You can also shoot the stickybomb if you're a gamer, but that's kind of hard.

Disco Inferno
All bosses who's name isn't Butcher (and maybe Rotjaw) are weak to fire, but the Assassin absolutely hates it. If he's in fire he'll run away, but if he's touched by fire he'll be staggered. Because of this, there's a silly interaction you can do. When the Assassin is about to enter frenzy set yourself on fire, but not him, and then run at him as a burning man. Keep holding the button to extinguish yourself so you don't lose health chunks, but letting go and starting again right before it finishes so you don't actually stop burning. He'll keep taking damage and stagger from the fire, and scream four different times. When he screams the fourth time he'll drop down and take full damage, so hit him with all you have there, and probably put yourself out too. During this entire interaction teammate(s) can be wailing on him, but he'll take less damage from those attacks.

Resistances: Poison
Weaknesses: Blunt, Fire, Explosives, Nitro Express
Weakpoint: Scary head thing
Frenzy: Just freaking out

Just as Butcher takes almost no damage from fire, Spider takes almost no damage from poison. He's also very evasive and non-standard like the Assassin. While he can still be hit with sticky bombs, throwing axes, and throwing knives just fine I don't really like throwing knives or axes at him since he's very evasive and a missed shot usually means it's stuck in the ceiling forever. He can spit poison clouds and has a tackle attack which knocks you down, extremely dangerous if there's hunters nearby.

Resistances: Explosives, Fire?
Weaknesses: Piercing, Poison, Explosives, Nitro Express
Weakpoint: Inside of mouth
Frenzy: Ferocious icky goop attack

Not sure if Rotjaw takes more or less damage from fire, but they spend most of their time in the water so most sources of fire wouldn't really work even if they are good. Her health is very low and weakpoint very difficult to hit without taking damage, so I'd recommend not usually going for it unless you're supporting someone else who's fighting her.

She has an incredibly powerful attack that does like 130 damage and knocks you down, so you NEED to have full health when fighting her. If you're missing even a small health chunk and think you're fine, then she hits you with that, you're dead. While technically possible to kill her without ever getting hit by her, I strongly advise you to not be out in the open where hunters can shoot at you from 300 yards away while you're missing a health chunk and fighting a boss in the loud, slowing water who has the ability to oneshot you.

Fighting Rotjaw is pretty simple. Walk in the water until you see her preparing for an attack, then sprint laterally (this means to the side). You'll dodge her attack, and can counterattack. If she makes a rumbling noise before the attack it means it's the 130 damage knockdown, but you don't actually dodge that differently so all it means is you really should definitely not get hit. After awhile she'll shed her collar and leave icky goo all over the floor, and if you stand in it it kind of hurts but mostly just takes all your stamina away. She also periodically does a little swim thing which looks funny. Just hit her with whatever during that. If you're too close to her when she does it you take damage, but it's not really avoidable.

With throwing knives & axes I recommend just immediately pulling them out like against the Assassin, and if you can sneak up on her before she enters the water you can try to oneshot her with a bomb.