The Arx fight I was referring to is after Kemm's garden.
Alexandar in the horror sleep arena fight, casting Ritual of Madness: I tried a save that would crash when he used the skill the next turn, but having a character summon an incarnate nearby would get him to switch targets to the incarnate, which wouldn't crash (don't know if that was just switching targets that helped, or the incarnate vs character specifically); one person with this problem reported being able to CC/focus Alexandar to kill him before the turn he would use the skill, and when he respawned after that, it did not cause a crash when he cast the skill