He is a Demi-Lich - it comes with the job. You will also notice that he casts Imprisonments (even originally) without the usual pause. I don't mind the rulebreaking in this instance - a Demi-Lich should be able to do some stuff that your regular casters can't do. Besides, once you prepare for the time Stop, it's not so bad.
Okay, I have no problem about the Demi-Lich Howl, or the Imprisonments. But still... Chaos in a Spell Sequencer, and Time Stop in a Chain Contingency?
Then, I decide, heck, I'll just get Keldorn, have him with the Firetooth +4 to kill Kangaxx along with my Ranger/Cleric with his Sling of Everad +5. No can-do. He regenerates insanely fast. Even when I CTRL-Q'ed him, and removed his Ring of Gaxx, he still regenerated something like 1 hp every 2 seconds. Combine that with the Ring of Gaxx, and both of them, couldn't do anything at all to him. Even with 3 hits with the Firetooth, and 2 hits for the Sling.
I really can't see a way past this other than a Dragon's Breath, or something like that. I need a weapon that's ranged, and that's +4. Reason for that is his countless Remove Magics; None of my characters can go close to him. I could use a Spell Immunity: Abjuration, but then he'd just switch target lock (yes, it has happened before). One character can't damage him enough to take him down. Of course, I could use a Scroll of Magic Protection, but I never really liked using them; they kind of take away from the fun of the game. I mean, what's the point of trying to buff up for a battle with Mages in case you can just use a Scroll of Magic Protection?