You can beat them with a single flaming sphere. They spend all their special attacks and invisibility potions for fighting the sphere because AI in this game is dumb af.
Larian thinks they can make the game harder by throwing more ac and hp at enemies but in actuality they make their own game more boring and unpleasant to play.
In multiple playthroughs, I've never had them focus on the sphere and not just break the caster's concentration and move on.
