This is a bit off topic because you don’t use her. But there’s a javelin build with karlach that carried me from early act 1 a good ways through act 3. Theoretically should work on any strength character, but best with a barbarian:
Subclass her into the frenzy barbarian. Take tavern brawler as your feat. Find the ring of flinging from the merchant in the Grove.
The javelin base damage is 1d6 + str damage, ring of flinging adds 1d4, and tavern brawler adds your strength again to the attack roll and damage roll. Plus the feat gives +1 str to bump karlach up to 18(+4 modifier).
Then, frenzy allows you to throw a second (and later third) javelin with your bonus action.
Each hit tends to have 95% chance, has considerable range, and reliably puts out about 15/hit with the potential to hit like 28 Twice per turn.
The frenzy hit also has a good chance to knock your target prone for that extra salt.
Then you can buy the pike of returning from the goblin merchant (must be before you agro them) to automatically get it back after you throw it so you don’t need a ton of javelins. And you can also collect the gloves of uninhibited kushido from the myconid colony to bump the damage up even more.
Casting enlarge on her helps with that damage output, as well as an elixer of the Hill giant, or anything that bumps str up more.