I suck at this game, played on Tactician, and didn't have too think too hard on this fight either.
How I approached it, IIRC:
- invisibility on team
- sent Gale to AoE the little brains
- sent my sorcerer to AoE the undead
- sent Astarion to free Nightsong
- sent Karlach (respecced to warrior) to instagib the mindflayer
Proceed to jump over to Ketheric, maul him, then play the 2nd phase with almost full resources.
