I remember the Illithids in Baldur's Gate 2 were a complete nightmare; if they mind-controlled your main character, it was basically game over.
Strangely, I remember there was also a certain item you could get early in the game that would make you immune to psionic attacks and powers, and if you had that they were easy. Just give it to your best fighter, hold back the rest of your party, and have them run in and beat the tar out of the Illithids.
Also, isn't there some D&D lore that they are actually aliens from space or something?