To beat the demi-lich (or whatever it is) I had the same problem. Simple solution: Make sure it's not YOUR character that's right next to him. Send someone else in first and let them get imprisoned, you can release them afterwards with a spell (I forget the spell's name). After he pulls that stunt you're basically home free. You might want to bring 2 or 3 mages in to cast as much anti-magic & dispelling on the lich as possible.
Good luck.
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