I can vouch for making Karlach a throw Barbarian with the Returning Pike and that trident which also has the return property. Equip her with throwing equipment and the Tavern feat and you've got yourself an Olympic champion that can annihilate everything in her path just by throwing a spear.
Karlach as a throw Barb is so strong, especially because physics in the game are wonky so if you aim at an enemy and then move your mouse a bit above their head, the spear will actually drop onto them and crush them via gravity. Unfortunately this also means enemies won't provide experience if they're crushed by gravity, but it's insane damage that's clearly not intended

Personally though I love making her a Wild Magic Barb with the Sussur Greatsword, +10 damage from Great Weapon Wielding and boots which reverberate the target on a condition. She silences, reverberates (reduces their attributes for each stack) and can knock them prone after 5 stacks of it. Really fun and helpful for everyone in the party.