Sneak attack: (requires 2 melee attackers focusing on 1 enemy). Works with ranged attacks and ranged touch attacks as well as melee. Every weapon focused character in the game benefits greatly from getting 1 level in a class that gives sneak attack and then picking the "accomplished sneak attacker feat"

Pets: create a zoo or undead army (summoning/class pets).

Kiting: If there is a slow enemy, chances are you arent intended to facetank him. Run circles and shoot him.

Mirror image spell: Mirror image (rank2 arcane spell) is one of the most powerful spells in the game and allows anyone with it to increase their survivabilty by a huge!!! margin.

CC spells: Bard and wizard/sorcerer with grease (rank1 spell) and stinking cloud (rank3) in particular
Damage spells: Evocation school (mage especially)
Necromancy is a notable all around excellent option for clerics.

Selfbuffs > Debuffs because they cant be resisted.

Lategame feats: There is a feat called "shatter defense" which allows you to hit attacks much more easily and apply sneak attack (treat opponent as flat footed). And there is a spell called "frightful aspect" that full caster divine/arcane classes get which passively applies shaken in an aoe with no saving throw (can still be immune). This is an extremely potent combination in the lategame.

You should also try to pick up the "blind fight" feat around the early midgame ( after the end of the troll chapter ) for several reasons.

Enemies attack whatever is closest to them first. If they miss the target too many times they switch targets. (Very few exceptions!)

There are the following types of armor:

natural
"general body armor" (The actual armorpiece you are wearing, refered to as "armor" in game)
deflect (rings and a few others)
shield

Each of the above stacks with each other but only the highest one of a category applies.

special type "dodge" (stacks unlimited, does nothing under certain circumstances)
special type "enhancement" (stacks once with its base type and with all other enhancement types)

E.g. The shield spell wont stack with an equipped shield. The mage armor spell wont stack with equipped body armor. The "armor enhancement" from the cleric spell "Magical Vestment" will stack with equipped body armor.

Dodge AC from dexterity is limited by how much dodge bonus your body armor allows and by how heavily your character is encumbered.

Notable:
Small size gives a minor armor bonus.
Halflings (level1) and Aasimar (level10) get race specific feats that give bonuses to AC.

I would highly advise against getting weapon focus early on your first playthrough or playing things that only allow 1 specific weapon. Picking the best weapon available is better pretty much all the time.

*updated for "definitive Edition"
The definitive Edition overall introduced more weapon variety so weapons that used to be barely in the game show up at least once or twice.

Most common types are:
Dueling sword
Composite Longbow - Devourer of metal is a godlike composite longbow Troll fortress, perception check required

Somewhat common types are:
Crossbows
Scimitar
Dagger
Kukri

The loot locations for these are less obvious though and you might miss some of the best ones in these categories if you dont know where to look/make use of artisans with the correct requests.

notable: longswords, slings and darts are extremely uncommon.

High persuasion skill on your main character.

At least 1 character with absurdly high perception - preferably multiple. Perception is the only skill where having multiple characters with a high skill value is consistently useful. Perception checks are not only used for hidden goodies but also to find locations on the map when walking along roads.

Delay poison or other anti poison methods. (use delay poison on your frontline BEFORE you fight monsters that can poison you. Spiders and Centipedes)

12 or more rations when entering a major dungeon.

Precise shot feat on ranged characters. It removes the -4penalty for "shooting into combat". (requires point blank shot feat).

The haste spell that pretty much all arcane casters can pick up as a rank3 spell is one of the most significant dps increases in the game for weapon characters.

Oleg sells a bag that allows you to carry more weight for 2500. Get it! Bring rations for dungeons!

Gloves that give +5 to trickery. Trickery gloves sold in your capital.

The game will still fail if you put kingdom on auto but dont finish a major quest within time.

Major quests will give you hints on where to start looking. Turn on location names (top right) on the world map to find locations you are supposed to search.

The best thing about unlocking areas and building cities is getting access to artisans who produce magic items for you. Kamelands, south and north Narlmarshes unlock 2 artisans each - you probably want to prioritize these. A quick overview of artisan locations can be found in the kingdom guide (link below).

You can buy BP (Build Points for your kingdom) from Hassaf in your capital.

Only start projects that require your main character if you have dealt with the current major questline (14 day advisor upgrades).

Advisor upgrades are the most important projects since they increase the stat bonus for the Advisortype.
Advisors use primairy stats (e.g. strength for militairy) to determine their individual bonus. (E.g. You have Regongar with 22 strength for a "+6" bonus and militairy at rank III (+3). Then you would get a total +9 bonus for solving events with regongar as militairy advisor.) A comprehensive list for all advisor stats can be found here:
https://steamproxy.net/sharedfiles/filedetails/?id=1534683229

There is an option in the difficulty menu that allows you to heal all afflictions (like stat damage and blindness) on rest. Another option resurrects fallen party members automatically after combat.

You only need 1 person for each skillcheck. The highest will apply automatically and you can choose when it doesnt. However there is a fairly significant number of persuasion checks in particular that are main character only.

There are various named "type bonuses" in the game for various stats (circumstance, morale and divine to name a few). Just like with armor - only the highest one of a type applies for its category but they stack with "type bonuses" that have other names.
E.g. You can have +2 morale bonus to damage, +2 morale bonus to attack (to-hit) and +2 divine bonus to damage at the same time.
But if you have +2 morale bonus to damage AND +3 morale bonus to damage then only the highest one applies(+3).

Nature Lore allows hunting. You can always hunt to save rations when you are not in a dungeon. You might have to turn off rations every time you enter a new map/overworld though (top left -> manage during the camp menu). Many vendors have rations but they dont refill.

Mages and archers (without a special feat, all enemies in general) will give melee attackers a free "attack of opportunity" when casting spells/shooting their bow.

Characters who are "flat footed" are vulnerable to sneak attacks and dont get their dodge bonus to AC. Every character in a fight is flat footed until his first action.

The %spell failure chance from armor only applies to mage archetypes (e.g. wizard,sorc,bard etc.). Clerics/druids can wear full plate and cast without any penalty.