Absolutely love this game but this needs a fix. Whenever a fight begins, I have like 20 archers instant focus on my main char (a backrank cleric) and in the later levels their attack bonus and damage kills him so fast. So either I have to buff him up completely even when just fighting mediocre bandits or I run away. When I run away the archers brainlessly rush behind him through the middle of a massive melee and just get butchered up completely by attacks of opportunity, instead of just staying in their good positions and firing at someone else.

I'm going to assume your formation has tanks in the front and your more fragile characters in the back of the group. Have you tried scouting? After you see what's ahead of you, you can send in your tanks first, and your squishier characters can trickle in to the battle piece meal.

Archers enemies like to target the character with the lowest AC, specially Pitax archers. Is you Main character the one with the lowest armor class?

Yeah, 100% enemies will always choose to target the easiest to hit enemy that they can see. If your backrow is squishy, either your front row needs a doorway chokepoint, insanely damaging AoO, or your back line needs to stay off screen.

One thing I like to do is charge my main tank into combat and leave many other characters off screen until the enemies roll initiative and choose a target.

Enemies pick the easiest target they can in range. So just don't be in range of enemy ranged with your ranged. Shoot their melee instead. Done. Also mirror+G.Invis are still good for your archers since you can reduce hitchance from 95% to 23.75%. Not exactly 1.25% proper tanks can get, but still a lot better.

Thanks for your answers! Ok, so the AI choosing the lowest AC enemy seems fair enough to me, I just assumed they always pick the main char for being the main char's sake.
I just came back to Kingmaker after some months and picked up a pretty advanced save, so my tactical shortocmings due to being out of shape may have caused that little rant. I will implement your suggestions, also had good experiences with greater invisbility.
Thank you all for giving calm and considerate replies, this defnitely cooled me down.

This game doesnt have an aggro system and enemies tend to focus "the weakest link", so make all your party tanky, even backrank clerics/sorcerers/etc. It's not even hard to make cleric tankier than warrior.

I'm going to call BS on that 'easiest to hit' thing. I'm playing against a horde of zombies and all of them just walk right past my flat-footed casters to attack my PC with the highest AC in party. I mean, great, most of them won't hit anything, but it feels like I'm playing with a toxic DM that's just trying to kill my character.