Don't know how many times I got killed till this time by the Demi-Lich (the floating skull); I'm boosted with spells and potions to the absolute max, got magic resistance at 50, but Kangaxx just imprisons my whole party (even though this is impossible since the maximum of memorized level nine spells is 2); How can you beat him???

Hopefully by doing this I can make it up to you.

Right, summon loads of monsters and woodland beings and get them to surround him.
That way (it worked for me) they should disrupt his spellcasting. Meanwhile, hang about at the back and hit him with +1 arrows and spells.

And pray like buggery.
It took me about 20-odd goes before I worked that one out.

Here's a little tip about demi-liches:
1. They are powerful
2. They can only be hit by +5 weapons (unless you are a paladin, then 4), or something like a mace of disruption ;).
3. i wouldn't even try attacking him until you find one of those weapons above.

hey hooligan, pals again ;)!
Anyway, the demi-lich is awfully simple to kill; just protect the first two characters with protection from magic scrolls and let your paladin whack him with the mace of disruption (located in the vampire lair); he'll just cast continously imprisonment on the one attacking him; that's it!