I just did it last night, on my *first* serious attempt...

After I completed the fight (and saved, fortunately) I reloaded and tried twice more... and got my butt kicked, lol.
The funny thing is the first time I did it, I did not use many of the traditional protection spells like bless, chant, defensive harmony, etc. Just good ol' hack & slash, with liberal doses of magic missiles, dispel magic (esp on the lich) and assorted other spells. I *did* have good weapons though, namely Celestial Fury and the Holy Avenger.
The beholder too did not seem to have much affect on my party, and no, I did NOT use the Shield of Balduran.
Oh well, I suppose I was lucky with some of the dice rolls, lol.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc