The Improved Illithids are indeed a pain. Nevertheless, they are a fun challenge.
Luckily, you are a Mage, and Mages can deal with anything.
Drink a Brine Potion to prtect yourself from their Psionics attacks, and Protection from Magical Weapons to render their tentacles useless. [img]smile.gif[/img]
Skeleton Warrior and Mordekainen's Swords are great summons agaist them - just be wary of Death Spells.
Oh, and one last piece of advice - be sure to memorize True Sight. The Improved Mind Flayers have a tendency of using Invisibility a lot.