Enter The Pit from the Hall of the Pit, and stay close to the door when you get in, and immediately hit Return to enter TBM. Find the nearest Queen of the Dead, and cast Paralyze at her until she freezes. With any luck, she won't have had time to cast Pain Reflection - this is the spell that will severely damage your party. Concentrate on her with all your ranged attacks until she drops. If anything gets in the way, kill it with minor attacks and switch your major attacks to the Power Liches. When your party gets badly damaged, retreat to the Hall of the Pit, rest and heal. Repeat as necessary. If you can kill one big monster at each visit, you can clear the place, although it might take some time.