stategy spoilr !
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its usually easy enough to beat the lich int demi lich form. put some traps down where he spawns if you have a thief.. then cast protection from magic scroll ON (*ON*) the demi lich form. this makes for constant spell failure for poor old floating skull.
beat him down with melfs minute meteors, holy avenger, staff of rhynn, or whatever.
spell immunity - abjuration is also handy to beat getting imprisoned-which somehow he can cast through the protection globe. the best thing about this strategy is you get to keep your other protection from magic scroll for later.
cheers.