Edit: Latter half of Act 1*

I've had an AC of 20 in the NWN 2 original campaign for the latter half of it. For whatever reason, my Fighter keeps on getting hit, and hit hard, by all creatures. It's getting very annoying when I put my melee focused person next to two guys and consistently have to retreat them, or better yet watch them die from 7 consecutive hits that wipe out the 110 hp.

Am I crazy or do the mobs get a bonus against your main character?

20 is pretty bad AC for the middle of the game.

I only have a save near they end, but near the end:

My AC was 42. You should have built yourself some nice Mithral mail, and have all kinds of nice gear to go with it.

Ohgren, using only found gear had AC 37.

Yeah I realized that I forgot the "ACT 1" part of my problem, haha.

Even at the end of act 1, 20 AC is low.

Base AC is 10. Full plate adds 8, dexterity adds 1, large shield adds 2, tower shield adds 4. So even a fighter with NO magical equipment whatsoever and no shield would have an AC of 19 as soon as he can buy full plate. 23 if he uses a tower shield.

By the time act 1 ends, you should have a bunch of magic armour, magic rings, magic boots, and magic amulets that boost your AC closer to 30 than 20. If you don't have those things... well, start checking out the shops! Get everything that boosts AC. Buy better armour. Get a shield, if you want your character to be a tank.

You can also look into the item crafting mechanics, though those are rather... convoluted in NWN2.

Also, buffing spells! Buffing spells are an essential part of NWN2 gameplay, and without them you'll be much weaker. That same AC 19 shieldless fighter without a single magic item I mentioned? Have Qara cast Mage Armour on him and whatsherface the Druid cast Barkskin, and suddenly his AC goes up to 27. Activate a brooch of shielding and you get 31. Now we're talking. And those are level 1-2 spells. With stuff like Improved Mage Armour, Tortoise Shell, etc, the AC bonus becomes even bigger.

There are also other ways to defend with spells. Cast stoneskin on your fighter, and he won't get hurt even if they do hit him. Cast Improved Invisibility or Blur, and some attacks will miss regardless of AC.

Offensively, buffs also make a huge difference. Bull's Strength, Heroism, Flame Weapon, the works: suddenly you'll hit twice as much and half again as hard.

As for the NPCs getting bonuses: you can see exactly what attack bonus the enemies get by reading the combat feedback window. If it says something like "Bandit attacks : *hit*: (11 + 12 = 23) then it means the bandit rolled a 11 on his D20, had a +12 attack bonus (due to level, strength, magic weapon, buffs, whatever) and it was a hit because his total score was 23 and your AC is only 20.

Generally, it's a good idea to pause the game regularly to see what kinds of attack bonuses the enemy gets. To be almost completely safe, you need an AC 20 points higher than the +X attack bonus the enemy gets. If your AC is 15 points higher, you're doing pretty good and will only get hit 25% of the time by their first attack, but a few more points of AC will drastically increase your survivability. If your AC is 10 points higher, you'll be hit quite a lot. If your AC isn't higher than the enemy attack bonus... well, I sure hope you have a lot of hitpoints or a stoneskin spell.

I took a look at my save when i finished ACT 1 and here are my stats.

My main, Druid Level 12: AC: 29
Base: 10
Chainmail +2 : 7 (5+2)
Dexterity: 2
Ring of Protection: 2
Shield (Dwarwen Mirth): +2
Qara's Mage Armor spell: +2 ( 4-2 because of CM +2)
Bark Skin: +4

Khelgar (Level12 Fighter) AC: 32
Base: 10
Full Plate: 8
Dexterity: 1
Ring of Protection: 2
Heavy Shield +1: +3
Qara's Mage Armor spell: +4
Bark Skin: +4

So yes. Your 20 is very very low especially for a fighter.