Recommended minimum level, 15. Mainly for one reason, Skeleton Warriors. I would advise that you have Project Image as well. When you enter the room, cast Project Image and spam as many Skeleton Warriors and Spider Spawn as you can.
Once you have casted all your protection spells, step forward and trigger the Lich's appearance. Once he starts summoning his friends, cast a scroll of Protection from Undead on yourself and concentrate on the Beholder first. A couple of Flame Arrows and Magic Missiles should take him out pretty fast. The reason for taking him out is because he has multiple Death Spells which will wreck havoc on your summons.
Leave the Mage alone, her own Pit Fiend will eat her alive. The Lich will leave you alone due to the scroll, so leave him for last.
Good luck, have fun.
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