Simply complete the first level by killing the bigger green dude and then using his key to open the exit door, which will be pink on the map. I personally recommend not worrying about ammo conservation or general attention to detail with this, as you will want to restart and purchase modifiers before most other achievements anyways. If you manage to find poison, get to low health and then drink it during level 2 so that you can go ahead and get that achievement out of the way. If not, feel free to continue, however I recommend letting the first monster kill you in level 2.

• Knowing what things actually do is normally pretty important in a game, however there is absolutely no description or help present throughout the duration of One More Dungeon, and unless you want to experiment around with every item you find I'd recommend just looking up whatever you find here:
  https://steamproxy.com/sharedfiles/filedetails/?id=577015178
  
  
• This game is a game of patience, health is the most valuable resource, and even getting hit once can be the difference between winning and dying on level 5. Take your time, check every corner, and never waste a health point just from prematurely drinking a health potion (unless you're one shot, in which case I made this mistake once during Curse on level 8, and I would personally advise never doing what I did, so at all times attempt to stay >4hp once you've reached level 6.)
  
  
• Loot everything. While I wouldn't advise looking in every room during your Curse run, in every other situation it's much nicer to find a weapon that doubles your damage than it is to beat a level 20 seconds faster.
  
  
• Play it sparingly if you're an achievement hunter. I've had this game for a long time, and only this year did I decide that I wanted to start getting the achievements in games. Some are more frustrating than others, and I have to say that this was worse than Cuphead or Dark Souls. While I was grinding the Curse achievement I only played 1 round a day, and occasionally I wouldn't even finish that round. At least in my opinion the ever-looming RNG involved in your success mixed with unforgiving everything makes it a bit annoying to play after you've choked the final levels even once. Just don't burn yourself out.

Changes the rules
Purchase the first modifier, [insert name here], and then select it for your next playthrough.


Elder Flower
Kill the Elder Flower boss on level 3. I would recommend getting a wall between the two of you, then shooting it with your wand while strafing in and out of the open so that it doesn't have any chance to shoot you.

Half way
Get to level 5 doing the same thing that you've been doing.


Ancient Necromancer
Kill the Elder Necromancer on level 6. The necromancers can spawn the skeleton ghost things right beside of you, but aside from that they're just standard ranged enemies, so strafe back and forth from behind a wall and they're not too bad.

Conclusion
Levels 7-8 are definitely the hardest. There are basically no healing potions, the enemies hurt, and in general everything is agony. My #1 recommendation is to watch your surroundings, accidentally stepping into a fire while backing away from an enemy can literally just be instant death. I would personally recommend something like the snowstorm staff or a higher grade acid staff that goes through enemies, because as you will see the bigger snakes split into 2 smaller snakes (also the level 8 boss is a giant snake, that then splits into 2 big snakes, that both split into 2 more baby snakes).

Finish the Game
Honestly the last level isn't too bad, just make sure you don't get ambushed by any of the creatures that start to spawn, and for the most part destroying the 3 portals is a piece of cake. Once you do this find the green portal, and congrats, you've beaten the base game!

Insects must die!
Just step on 25 of the spiders in levels 3-4.


Drink me
I'd recommend just doing this on level 1 during a run you don't care about. Take enough damage so that your health is already low, then just drink poison and watch your health slowly go down ¯\_ (ツ)_/¯.

Time is money
I accidentally did this during a normal playthrough, but it's pretty straight forward. For the most part you can run past enemies in level 1 (although that isn't really necessary), and just try to find the level boss/exit without worrying about any form of looting. With some good rng they can be directly beside of spawn. If you're really struggling, there are multiple artifacts that can help with this, but as I said I recommend just doing it normally as that will probably be faster.

Full set
This one just takes some grinding, but you should be able to afford all of the mutators just by getting the rest of the achievements. I'm pretty sure both of my completions gave ~20k coins a piece.

Trader
I got this achievement on my curse run, so technically depending on your rng/skill this could take some extra grinding since, were it not for a trader on level 8, I wouldn't have gotten it without going back out of my way to get tokens and find something to spend them on.

Full equipment
I'd recommend just doing levels 1-2 normally, then playing through/quitting out of every level past that point until it fills at least 1 new item slot. Hopefully it should at most only take 2 exits to get something each level, however from my experience you will never get this without actively grinding for it.

I can't see anything!
I would highly recommend doing this with the Broken Cartridge mutator, as fog genuinely can make the game much harder. For the most part this is pretty straight forward, spend level 1 getting used to the new color scheme, then just don't have a seizure while playing out the rest of the game as normal. I think the only genuine difficulty spike I noticed was against some of the ranged enemies/bosses, since if multiple were on the screen it could become hard to tell when their projectiles were flying at my face.

Bring 'em on!
I'm not going to lie, this one is a bit hard. I would definitely recommend playing this with curse, as it is extremely easy to accidentally get hit if you're using No Margin For Error. The most important factor here that you most likely haven't experimented with is exiting to the main menu. When you do this, it fully resets the progress on whatever level you're on, also meaning you come back with your original inventory/stats and generate into a new map once you continue. This can be used to try and get good loot, or you can exit out of the game whenever you're about to die and work on attempt #2 instead of having to fully restart. Outside of that it's all health conservation, with finding a balance between speedrunning, so that the poison doesn't annihilate your health, and careful calculation, so that you don't take a hit every 30 seconds just from the normal monsters. I would recommend resetting in level 1 until you can come out of it with at least 4 forms of healing potions, then try to get 3-4 more on level 2 since for the most part the later in the game you get the less you will find. There is a much more detailed guide for this here that helped me out with it quite a bit:
https://steamproxy.com/sharedfiles/filedetails/?id=868421770