Okay, I feel I should probably ask this here before I pop a blood vessel.
I've tried the fight several times, its a pain in the ass but I'm losing every time because when the Ulathirid, which I found nothing on in terms of documentation on from googling it, at least in this battle, is seemingly invincible and when everything else is dead, initiates a cut scene, that kills everyone. The only thing I could think of at all is try and ensure at least one character has protection from magic energy on, since that says it gives 100% invulnerability to magic based attacks which as far as I know in the game Psionics are considered that, but nope.
What in the hell are you supposed to do here, and why on Earth does no one talk about it?
*edit* - okay I might have a possiblity for what the issue is after watching a video, the thing is meant to die if you kill the rest of the Illithids, and maybe the problem was I was using imprison on one, I guess I will not do that and see if that helps.
**edit** - yeah... I got angry from my own stupidity. The mage is a pain in the fight but otherwise, not difficult.