Make your paladin a 2h smiteadin with a bard dip for college of swords maneuvers. Yes bring karlach and laezel and all 3 of you single-target 2h shredder your way through the enemies.
Assassin astarion is a monster and can usually sneak attack kill targets in one shot. Especially if you go the ranged route and add the extra base damage from that one ranged feat.
Once gale has fireball you can trade him out for astarion and just huck those things around as much and as often as you can and rest every time he’s out. He’s also really good at magic missile spam with those electricity items.
Most importantly, you should always prioritize killing one enemy at a time so that you’re quicker to remove them from the turn order and prevent them from doing damage on their turn. Basically never split damage with your single target party members.