The battle against the Master Brain, in the Illithid Lair is easyer than it seems. All you have to do is haste your party and then wait in the room before the corridor to the Master Brain room while your mage and you main character. Protect your main character(barbarian rage or that little potion that you get in the Illithid Lair) and open the door. Cast death cloud or death fog whith your mage and quickly close the door whith your main character. Repeat the operation untill all the mind flyers,Brain Gollens and Umberhuks die. Then open the door and slay the Master Brain easyly.

Actually, the Master Brain is a pushover. It is the minions that gives you 'trouble'.

I usually have a level 12+ Mage by that point in the game, assuming that I am playing a party. I simply cast one, yes, one Chromatic Orb and the silly Master Brain dies.

Skeleton Warriors and Blunt Weapons handle the Mind Flayers, Umber Hulks and the Brain Golems.

i just hasted korgan and invisible him and just hacked it down, not that tough...