Ah yessss, the good old Kangaxx problem
Tips to kill him are: The demi-lich can only be hitr by +4 or better weapons, Is immune to most magic (but NOT the Ring of Ram), he always tries to imprison the nearest target
Easy ways to kill him are: First kill the lich form, then use a scroll of magic protection (bought from the adventurers mart) on one of your characters who is carrying a +4 weapon (Staff of rynn is probably the easiest to get/use and I think it works), get that character to atack the demi-lich and Bud-a-bing, one dead Demi-lich
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Nicodemus Deathslayer, Paladin of the Order of the Holy Flame