Need advice for my party

So in BG3, my party currently consists of Gale, Shadowheart, Karlach, and my good old Berserker Tav.

Me and my sister are kind of racing toward the end of the game. This is my first playthrough, and my sister wanted to play with me, so I told her I still need to learn how the game actually works.

Here is the thing: my sister’s boyfriend said her Karlach deals at least around 60 damage per turn with two attacks, while mine usually deals maybe 50 damage max, even with around 4–5 hits in one turn.

I am not trying to fully min-max my party or break the game, but I do want some advice. How is her Karlach dealing that much damage so consistently while mine feels weaker despite attacking more?

For context, I am currently in the Underdark, and I killed the undead wizard guy who has that huge creature named “Meat” with him.

What would you suggest I do to improve my party's overall damage?