From my testing, skeletal kings need 25 willpower, wyrms 30, and minotaur mages/emperor liches/possibly keriax(not tested) 60 willpower to be immune to their confusion attacks. Learning rings give you +5 effective willpower and clear mind rings +10. I've been told that my testing is mistaken on the latter part of this though, and just today I had a clear mind (unblessed) with 20 willpower and was not immune to a red wyrms confusion attack, so I must have this not completely right.

I think with water orb (30 will) and two clear mind rings it should be possible to fight keriax or minotaur emperor without confusion worry (to the point that you might spend a wish on one of these rings).