This game uses turnbased combat.

By default All actors can perform 1 move action ( dependent on speed ) and 1 (special)attack/cast spell/use belt item/interact with object/switch weapon sets. The game calls that an "action". One can get and extra "action" per turn (with reduced initiative) using the lvl 4 of the spell "Move as the Lighting" buff.

Speed parameter (alterable via buffs/debuffs during combat) controls the maximum number of hexes for movement during a turn. Similar system with Firaxis's XCOM series, only better since it allows you to manually path (requirement since pathing in very important in this game - floor plate triggers, hidden/detected traps, enviroment hazards, attacks of opportunity, etc )

Base speed value 8.

Warcraft talent at any tier provides +1 speed.
There is a belt item that provides +1 speed. (mumified toe Duyar)
Various spells can affect (movement) speed Cold Shock, Move as the Lighting,..
Wounds reduce speed.
Some potions & posion debuffs (consumables) can affect speed.

The combat turns are simultaneous: both your party and your opponents get to act during the same turn. The order of "acting" in combat is governed by the initiative stat/system.

Default initiative value is 7.

Extra initiative sources:
(abilities) Battle reflexes + 4 initiative. Battle intuition +2 Initiative. Battle dancer +5 Initiative.
(on weapons) The initiative modifiers on both weapons (dual wield) are added. Dual dagger can really pump up initiative. Dual bashing weapons can really eat into initiative.
(various spell buffs and debuffs and potions) Few things can ruin your day like a mass cold shock debuff.
(perception talent provides initiative boost from tier 2)

This is a critical game mechanic so I must explain it a bit.
Significant changes from BG1.

Base Chance to hit 60% for melee. 50% for ranged weapons. (if attacker's offence - target's defence = 0).

THERE IS SINGLE tohit d100 dice roll for every attack (even special ability attacks that hit multiple times like "triple shot" and "twohanded attack" with potentially poisoned weapon(s) ).

NO MORE CRITICAL FAIL on roll "19-20". YOU CAN NOW REACH 100% chance to hit with both melee and ranged attacks assuming enough offence - defence.

Base chance to crit for melee and ranged attacks is ZERO. You can increase it SIGNIFICANTLY from skills and gear but the overall effect has been nerfed (see damage formula bellow - it now only doubles base weapon damage and doesn't double the rest of the damage bonuses anymore).
(SPELLS DON'T IMPLEMENT CRITICAL HIT/FAIL mechanic)

Sources of Chance to Crit (CTC):
-weapon talent (from +5% on bows to +20% daggers)
-on weapon special , a few some weapons come with +5% or +10% CTC or +20% CTC vs enemy type
-weapon master/master archer special abilities +10%
-spell buffs sensatocco masterstroke + 0/5/10/15 %; hawkeye marksmanship + 0/5/10/15 %


Melee CTH: [Effective_Attacker_Weapon_Offence - Effective_Defender_Defence + 60%]*[1-savingthrow] //60% CTH when EAWO = EDD )
Ranged CTH: [Effective_Attacker_Weapon_Offence - Effective_Defender_Defence + 50%]*[1-savingthrow] //50% CTH when EAWO = EDD )
Saving throw is 0 by default and can be increased temporarely via the spell duplicatus double vision (20%/30%/40%/50%). Extra multiplier lowering the chance to hit.

HIT RESOLUTION (CTH AND TO HIT DICE ROLL):
Calculate main hand and optionally off hand CTH and CTC. Clamp them between 0 and 100.
Roll d100 dice.
If (diceroll <= main hand CTH) then MAIN weapon hit.
{
If diceroll <= main weapon CTC then also a crit.
Apply main hand primary damage, special damage, built in poison damage&debuff, temp poison damage&debuff.
If (diceroll <= offHand CTH) then OFF hand hit.
{
?? Check for offhand CTC ??
Apply off hand primary damage, special damage, built in poison damage&debuff, temp poison damage&debuff.}
}
Else
{
If dodge was used print dodged (used up target's dodge defence bonus for the turn)
If parry was used print parried (used up target's parry defence bonus for the turn)
If neither the above print missed.
}

Yup, 0% CTH will also consume parry and dodge attempts .. High initiative way out of range ranged attackers will often do this to your characters.

Effective_Attacker_Weapon_Offence (CAN GO BELLOW 0) is a sum of:
-base +40%.
-weapon talent offensive slider (between 0% and 70% for melee weapons; between 0% and 100% for ranged weapons).
-weapon talent tier bonus (some weapon talents provide small offensive bonus at various tiers).
-weaponmaster melee/master archer special ability +10% .
-perception talent tier 3/4 provides +10%/20% ranged offence.
-special maneuver offensive penalty.
-on weapon offensive modifier (can be negative).
-SPECIAL (blue) on weapon offensive modifier +10% for off-hand (lefthand dagger, small tuzak blade)
-on shield offensive modifier (usually negative) for main hand.
-shield fighting special ability +10%/+20% for main hand when wearing shield in offhand.
-effective off-hand penalty for off-hand weapon when dual wielding (-60%/-40%/-20%).
-belt item +20% offence for melee attacks Tairach Amulet.
-bellow optimal range for ranged attacks -100% (can still achive 100% CTH with enough bonuses to offset).
-past optimal range for ranged attacks -10% PER TILE PAST MAXIMUM RANGE.
-low obstacle on LOS to target -30% for ranged attacks.
-target in viable cover (mode) -?% for ranged attacks.
-spells/potions/poisons/wounds offence buffs/debuffs. (frenzy/LFY etc etc loong list)
-story bonuses for mercenaries
-etc..

Parry defence bonus applies at most once per turn. Cannot parry ranged attacks without shield.
Dodge defence bonus applies at most once per turn. (HUGE NERF compared to BG1) .

Effective_Defender_Defence (CLAMPED AT 0) is a sum of:
-weapon talent defensive slider (between 0% and 70% for melee weapons; 0 for ranged weapons).
-on MAIN weapon defensive modifier (can be negative), irrelevant for offhand.
-on shield defense modifier.
-belt item +20% defence skeleton turtle.
-story bonuses for mercenaries
-spells/potions/poisons/wounds defence buffs/debuffs. (frenzy/LFY etc etc loong list)
-best of still available dodge and parry bonuses that build on top of defence but only for 1 attack per turn:
--weaponmaster melee/master archer special ability +10% (parry/dodge).
--2H SWORDS weapon talent tier 3/4 parry bonus +5%/+10%
--dodge 1/2/3 special ability +10%/+20%/+30% dodge
--spell standfast 1/2/3/4 +30%/60%/90%/60% mass dodge

As you can see IT IS EXTREMELY DIFFICULT TO RAISE DEFENCE WHEN WIELDING RANGED WEAPONS.
SHIELDs are very usefull for buffing both parry and dodge and maintaining some defence after those are expended for the turn.


Example of a decoy character build (very hard to hit with MULTIPLE melee and or ranged weapons)

weapon slider +70% defence.
weapon (small tuzak blade, captain's sabre or lefthand dagger ) +10%
shield tower shield +30% ; blazon shield 25%
belt +20%

Total 130% unbuff/debuffed defence.

A naked character with no skill starts with 40% base + 60% = 100% (0% CTH this char).
Getting 70% from weapon talent (tier 4) gets him at 170 - 130 = 40% CTH regular attack vs this char. (decent)

True for melee and ranged regular and special maneuvers.

(BaseWeaponDamage * CritMultiplier * SkillMultiplier + Sum_of_Weapon Damage Bonuses)*(1-effective_relevant_resist) // Rounded up

BaseWeaponDamage: weapon damage listed on weapon info screen.

CritMultiplier: 2 if crit , otherwise (nerf from BG1 it ONLY applies to base weapon damage on BG2)

SkillMultiplier: 3 if attack was hammer blow, 1 otherwise (nerf from BG1 it ONLY applies to base weapon damage on BG2. Felt harder by single handed hammer blow use that will do disapointing damage when encountering resists due to the lower weapon base damage. Not multiplying the bonus damage anymore makes hammer bow damage output drop behind dual wield most of the time when a lot of bonus damage is involved since dual wield effectively doubles bonus damage).

Sum_of_Weapon Damage Bonuses:
-weapon talent tier from +1 to +3 (see bug report for dual wield https://steamproxy.com/app/314830/discussions/1/135511379836970502/)
-special weapon maneuver damage bonus (+5 power blow, targeted stab, death blow, targeted shot)
-weaponmaster/master archer +2
-belt item Bloody Claw +3
-on Weapon( +X damage vs enemy type). Stacks on both weapons from both weapons. (ex great bone mace + bone mace both get +10 damage vs magical enemies when dualwielded. Both Assassin's "dagger of the crows" custom daggers get +10 damage vs humanois ; both dual hunting knifes get +6 vs animals).
Much weaker bonus on two handed weapons that can apply this bonus once compared to dual wield that can appliy it 4 times per attackaction.
-on ranged ammo +4 (war arrow/bolt)
-spells & potion buffs (+2/+3 melee sensatocco masterstroke; +1/+2 ranged hawkeye marksmanship; +5 melee weapon balm and berserker potions; +2 ranged target potion)
-hard difficulty bonus for enemies +3 for ranged and melee attacks. Your mercenaries will also get this in the training fights.
-etc?

Damage resists can be both buffed and debuffed during combat (paralyze, fastness of body, witch bile tier 2+ spells). They can also have negative values on some enemies causing them to receive extra damage.
(sadly no way to read them directly in game for enemies - only via reverse engineering the damage formula they can be painstakingly measured).
Positive damage resists will be set to 0 for targeted stab, death blow and marksman special maneuvers that "ignore armor".

Some weapons have extra damage type bonus special. That damage is applied sepparately like for damaging spells: (ex brabak lad +3 stab, flying kraken +5 fire, bow of the great hunt +6 magic,mage's staff +5 poison, etc..)
Damage * (1-effective_relevant_resist) .

Permanenty poisoned weapons also apply posion debuff (poison damage over time applied between turns).
Thas type of bonus is particularly potent on ranged weapons that can guard (3 attacks between turns) , triple shot *3 up to two times per turn vs single target with bows and arrow storm *7 up to two times per turn vs large enemies.
The champion of such weapons is the "Bow of the great hunt" (+6 magic damage) obtainable from one of Faramund's murderous pillage missions.

The armor damage reduction system has been clarified [ weapon and spell damage is multiplied by (1 - effective_relevant_resist) and then rounded up] and the interface does a much better job at displaying the various armor stat bonuses and set bonuses.

Without attributes there is no strenght and those GONE ARE THE INVENTORY NIGHTMARES OF ENCUMBERANCE LIMITATIONS OF BG1.

There are four types of armor: simple, light, medium and heavy.

Light, medium and heavy inflict progressively harsher stats penalties when used without appropiate training (they list the penalties well enough ingame so I can save some space here :P ).

The material (metal/non metal) also can inflict various nasty stat penalties ON CASTERS when attempted to wear without appropiate training (nicely listed in game). Mages can (with adventure point cost) use metal armors efectively in BG2. IMO the best "caster" armours are simple non-metal and thus require no extra training (game is short, adventure points are scarce BUT if you want options you got'em !) .

link to (inclomplete) google sheet with stats of weapons, armors and belt buff items I've found in my playtrough.
https://docs.google.com/spreadsheets/d/1Ok2ShQpMcQgeKMEmSQiEapsv4vLUU3UuWqtGTk1sbZI/edit?usp=sharing

The special maneuvers got quite a working over with a multitude of gameplay implications.

First off - the newly introduced endurance system. Special maneuvers consume tons of endurance and endurance is pretty hard to come by.
You can get it off skills and gear but it comes at a steep opportunity cost.

[ENDURANCE REGENERATION DIRECTLY COMPETES WITH ASTRAL REGENERATION ON CASTERS AND BY SHARE FLEXIBITY AND POWER ASTRAL REGENERATION IS THE SUPERIOR HORSE WHEN POWER GAMING IS CONCERNED]
For optimal "power" output - all characters that can cast spells, should !
Mixing spell casting and special maneuvers is still suboptimal since the game is short and the adventure points are scarce. (especially on a "good" alingned playtrough that restrains from burning "people at the stake" for extra loot and XP).

Puting at least some points (~50 before the AP cost per point goes up) into a weapon talent is a solid investment since it is the main source of defence in BG2.

2Weapon Strike:

(see BUG report https://steamproxy.com/app/314830/discussions/1/135511379836970502/)

The new overall "best" melee weapon maneuver. (IMO)
Is a special attack available when equipping a second weapon in the offhand. (Single to hit roll made with main hand decides critical fail/hit and if the main hand hits.?) Extra comparison done for second hand effective attack if needed. By default the offhand gets a -60 offence penalty. (in BG2 offence can get negative). Dual wield special ability adds +20 offence per level to off hand weapon.
Defence mod on offhand is irrevant but the initiative mod is relevant.


Shadows clone spell has been nerfed (cast range) and shadow can ONLY use "2Weapon Strike" with melee weapons and "guard" special maneuvers.
Damage bonuses add only ONCE vs a single target to any 1 handed attacks (after hammer blow nerf they aren't multiplied anymore) but TWICE for dual wield.
The newly +X damage vs enemy type on weapon stat "scales" much better on dual wield (bug?) than on any other special maneuvers since it applies twice to each weapon when dual wielding. (*4 vs *1).

Two separate weapon strikes so there is the potential for dual poison aplication per action.
(POISON DEBUFFS FROM THE SAME POISON TYPE DON'T STACK, they reset duration . ALWAYS USE 2 SEPARATE TYPES OF POISON ON EACH WEAPON). (Metaknowledge - always apply the poisons OUT OF COMBAT - saving the first turn action and freeing the belt slot) .


Hammer Blow:
The single target damage king has fallen a bit. The *3 multiplier is only applied to the weapon base damage (huge nerf - bonus damage applies twice for dual wield - once per weapon). Higher endurance cost than 2 weapon strike. Can get better offence and defence with 1H + shield but will often lack damage output. 2H version suffers from much lower offence that will force more buffs/ sacrificing the defence slider ...more at the mercy of RNG when CTH is not 100% .

Power Blow & Targeted Shot
The penalty is low enough and the gain significant for these attacks to carry the early game and remain situationaly useful later (tough targets, debuffs, etc ..).

Targeted Stab & Death Blow
Have been buffed a bit +5 Damage AND armor ignore can be significant. Wounds are much harder to resist. Still the damage output will rarely provide 1 attack action kill.
You can kind of get around the armor ignoring part by using (metaknowledge) THE RIGHT WEAPON for the job. Safe skill for purists at blind playtrough as they nearly guarantee decent damage output vs non-immune targets and allows for decent defence (shield use).

Liberating blow:
Has utility. Up to * 3 damage but split vs multiple targets. 1 CTH roll - all crit at once. The PUSHBACK EFFECT (when not resisted : targets roll body talent to resist effect. ) TURNS INTO AUTOKNOCKDOWN IF IT HITS OBSTACTLE, WILL TRIGGER ATTACKS OF OPPORTUNITY AND WILL 100% CTH for fallowing hammer blow). Can be used to kill/push enemies off ledges/docks.


Blade Storm Buffed but still not very usefull. If your're easy enough that enemies like to surround you'll be down before you get to act. Damage is low and to add insult to injury it on only applies poison debuff NOT the on strike poison damage.

Knockdown:
Buffed - by the nerfing of the body control talent + attributes that provided immunity to it. Solid utility - can shudown some nasty casters unlike in BG1.

Triple shot:
Single to hit check with -80 offence penalty. Poison damage and any extra damage is applied thrice on attack action. Uses 3 arrows. The BG2 king of single small target damage output.
Damage reduction is substracted for every arrow/damage application (can be a problem when using the "wrong" damage type of arrows).

Arrow storm:
The undisputed king of damage vs large enemies *7 returns. 1 Tohit check vs multiple targets. Up to seven times the poison aplication. No longer immune to range offence penalties.

Targeted Stab & Death Blow
Have been buffed a bit +5 Damage AND armor ignore can be significant. Wounds are much harder to resist. Still the damage output will rarely provide 1 attack action kill.
You can kind of get around the armor ignoring part by using (metaknowledge) THE RIGHT WEAPON for the job. Safe skill for purists at blind playtrough as they nearly guarantee decent damage output vs non-immune targets and allows for decent defence (shield use).

Guard:
Cheapish AP wise Cloned Crossbows magic users.

Feint:
Spend precious rare endurance (stamina) to do low damage and nothing (drain enemy stamina). Killed enemies often respawn on the spot with full stamina. Once your melee guys run out it isn't cheaply replenished. Can maybe be used to "declaw" some enemies you don't want to kill since they respawn just as new ?

Mechanics did not change much (at all ?) from BG1. Now interface show the stats of the coated poison on weapon.

"Poisons" appliable on weapons have up to 3 potential effects. Applied poison will be "consumed" from weapons in combat after (how many ?) attack actions.

-INSTANT damage after weapon hit, separate damage from weapon vs target poison resist (will not get doubled on crits). Dual wield can (should) apply two separate poisons.
-various stat debuffs (turn limited or for combat until cured )
-poison damage debuff - adititional poison damage applied at the begining of turns (turn limited or for combat until cured )

Poison resistance will reduce the INSTANT damage received on wepon strike and the poison damage ticks at the begining of turns. When hit by poisoned weapon the debuffs apply automaticaly to non poison immune targets.

POISON DEBUFF FROM THE SAME POISON TYPE DOESN'T STACK - they refresh durations.
Wastefull to use same poison type on both weapons - use different types to add multiple different crippling debuffs to the victim.

APPLY THE POISON ON WEAPONS BEFORE THE COMBAT. Saves first turn(s) action(s) and frees belt slots. (requires metaknowledge - posions are expensive and limited).

Hammer blow is one weapon strike. 1 poison application.
Dual wield - separate weapon strikes will apply (separate) poisons (also costs more).
Triple shot - 3 arrow hits - INSTANT poison damage applied thrice, debuffs overwrite.
Arrow Storm - potential for 7 arrow hits on big targets. MASSIVE DAMAGE POTENTIAL vs large targets.