Aha. Inexperienced you say. What items do you have? I
assume the Shield of Balduran and Daystar. Takes care of two
of the problems. Cast insect plague on the mage that appears
inside the room with you. Summon other allies to take the
brunt of the initial spells. Time stop, etc.. Finish off
the mage in the room. Use potions of invis., haste.
Gate in demon with Anomen, insect plague the main room. You
can always false dawn the undead, delorous decay.
Not necessarily all in that order but find some good stuff and
mix it up.
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If things got any better, I'd be twins.