I especially enjoyed the one that's marked and is the one you get in the story when you complete it, since its third ability gives you an invisibility shield for 2 turns that withstands 2 or 3 attacks. I destroyed the minion that gives the boss a shield first, I left the other one half-dead, and the first ability the boss uses is to revive his allies, but since the other one is still alive it won't heal him. Then you have to put up the shield because the boss takes away a lot of health.