I'm afraid I can't recall what happened and how exactly, since I only reloaded once (Minsc did get imprsoned the first time and I had no Freedom scroll on me at the time).
My party consisted of me (elven archer, dragon's breath halberd & heartseeker bow), Minsc (Dual-wielding Flail of the ages +3 and Arbaines sword (I think. It's the +2 free action giving shortsword), Valygar (dual-wielding his own sword and something else, also wearing the ring of the ram to provide the killing blow), Jaheira (staff of Rhynn +4), Aerie (Mauler's arm and a sling +2 or +3) and Imoen (armed with something something)
Minsc and Valygar were in front, backed up by Jaheira. Me, Imoen and Aerie were somewhere in the back. Now it might be that some weapons had no effect, and maybe the thing had magical resistance, but I would have remembered everything better if my party didn't finish off the Lich and Demilich versions in just a few minutes, using nothing but Hack 'n Slash and a couple of spells. (It either worked, or I might just have been really very incredably plain lucky
)
As for the "Really powerful tips" reply, these are only the main mayor lines of the strategies given here. Try doing a search for Kangaxx and you'll see (though maybe it's best if you didn't)
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The last arrow of Legolas kindled in the air as it flew,
and plunged burning into the heart of a great wolf-chieftain.
All the others fled. -J.R.R. Tolkien, The Lord of the Rings