There are a ton of strategies on the board for beating him. Many of them are very good, though no one has mentioned my favorite. I like beating up on the lich same as any other caster. When he makes his transformation, I beserk my main fighter (to counter the imprisonment) and pound away on him with the +1/+4 vs undead flame tongue. He'll spend all his time casting worthless imprison spells and will go down quickly. If you have a beserker in your party, you could toast the lich with traps and then beat the demi-lich with the beserker. You'll just have to be careful to keep the rest of the party out of site so that they don't find themselves in little bubbles far below the surface.
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