Made some tests on mind flayers in the Bodhi's dungeon, and they seem to have a decent magic resistance, which renders them nearly immune to Death Spell. And also to Horrid Wilting
No other enemy annoyed me more than these flayers. Finally I ended up shapechanging into an Iron Golem, imp. hasting myself and smacking them to the floor, but in their lair, they will be more of trouble than then. Is there a summon which is immune to their attack?