[Dace De'Briago]

Death Spell is pretty good against umber hulks, should kill most of them.

You may want to use Chaotic Commands as a buff spell to prevent your characters getting confused when they cast confusion at you...

Magical Swords and Skeleton Warriors are great summons to use against the mind flayers themselves, they are fairly poor in hand-to-hand combat - it's just the intelligence drain that is a pain. Note that protection from magical weapons works nicely as a defence from being hit.