You don't really lower a target's AC, it usually stays constant. The AC is just the number you have to roll against to hit.

Ie, you roll a 10 on a d20. You have a bonus of +8 to hit. That means you'll hit the enemy as long as it's AC is 18 or lower.

Saving throws are the reverse. The enemy who's getting targeted by a spell gets to roll and add their save bonus to try and beat your spell DC, which is usually somewhere from 15-20 ish.

You don't really need to know the maths going on, it's all represented by the % chance that pops up when you go to target something. If a hit says 95% chance, that means you'll hit as long as you don't roll a 1 on a d20.

As for Karlach's AC, that's just a barbarian thing. Barbarians are usually unarmored, but they get to add their Con stat to their AC as well as their Dex. They have a lot more HP to make up for getting hit more often, and they resist all physical damage while raging so they're still quite tanky despite having a lower AC.