I often forget that I'm playing an SCS modded game lately, but I was reminded last night when the party decided to tackle Kangaxx. Unlike in the past, where he would focus his attention on the party member closest to him, he now wanders around spreading destruction. Worse, he was casting PFMW about as fast as my sorcerer could drop it, and it became a race between them. Meanwhile, the party wasn't hitting him at all - even with weapons like Carsomyr, Firetooth +4, etc. Eventually, he would get off a successful imprison on a party member, and even with scrolls of Freedom, it was clear this was going nowhere.
So, I did a bit of research and had the party buy a set of Scrolls of Protection from Undead. I was warned that at least one party member needed to forgo protection or Kangaxx wouldn't be able to drop the dialog that indicates he is switching forms.
That went about as poorly - as soon as he saw one party member, he started attacking them all. They were certainly immune to physical attacks, drain, etc, but not to the variety of 8th and 9th level spells he was dropping - again, including imprison. Rather than focus on the one party member who was attacking him, he was ignoring Imoen and powerbombing the party members WITH protection. Imoen was literally chasing after him! It looked like he was making a beeline for Eilrie, the PC, so it's possible that she could have stood in a corner with spell protections and a death ward as bait, but that room is small enough that everyone was getting gassed.
One reload later, I had the whole party use the scrolls. This time, the encounter was a bit closer to cleaning up a pesky rat in the sewer. He just stood there while Sirene chopped him into golden cordwood with Carsomyr - the only input from Eilrie was the initial Pierce Shield. The whole battle was over in about 5 minutes. He DID address the party and change into his demilich form, so the party was able to get the ring, but the journal entry didn't get updated and still shows as pending.
The thing is, I'm not sure how any party could handle Kangaxx with SCS unless they were all mages, since you can't cast spell immunity on other characters. Death ward, sure, but that only partly helps - imprison is still a massive risk. Trying to keep him busy with summons didn't go so hot either, when his casting time is so short that he can insta-nuke all of your summons, even Mordekainan's swords. (I did have the party try peppering with spells to remove his protections and then sending a horde of swords his way)
All in all, a somewhat unsatisfying end to this quest. The first two liches were actually challenging without being insane. With buffs and protections, the party was able to tackle them. Kangaxx? It was either a complete wash or a walk in the park.