I played on normal, and while it was the first fight to last longer than couple of turns, it wasn’t too difficult. Wyll focused on killing the archers - before that he summoned a devil (granted by Mizora) to occupy mindflayers. My Tav bard cast haste on Karlach drank potion of invisibility and went hiding in the corner => Karlach andnShadowheart went on to murder non-archer minions, and than went to focus on Kethrck. Once he fell Karlach kept smashing the new boss with 5 attacks per turn, while the rest took care of the summons and focused on the boss if there was no other target.
I think the Apostole fell in 2 or 3 turns - I remember thinking that Karlach is about to go down, and was about to retreat her, but she burned through the remaining boss health.
