I got through kagnaxx but can not get past the demi lich. He just kills all my characters, any idea how to kill him?

Cast protection from undead on your party, cast all the summoning spells you've got, and just beat the sh*t out of him. If you're lucky, you'll get him.

You will need +4 or better weapons to hurt him, also it would be helpful if you have characters who can "enrage" (ie. berserkers like Korgan) since enraged character is immune to imprisonment spell Kangaxx uses.

Not only the +4 weapons can hurt this rags but also the Black Blade of Disaster can kill him for good.
I think your summoned beasts are useless 'cause they will be killed from the beginning of the battle by the demilich death spell.
I've tried it and I know. So don't spend too much time with these spells.
If you have no PC can become enraged or +4 weapons try to use your clerc or paladin to repulse undeads, that can decrease a little the casting rate of the demilich.
You can have one of your mages protected by a 9th level Protection against spell attacks stand near Kangaxx to attract his attention ( you should use scroll instead of memorized spell).
There's another way to kill him : change your form to slayer but that is not a good way.

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I hide in dark and I'll hit...