I've completed the other two parts of the quest for Pelagius III with Sheogorath... but I am utterly stuck on the atronach thrall battle. I got that I can use Wabbajack to switch the atro's form... but as soon as I get my atro to a form that is strong against his, he just immediately changes it to one strong against mine! I have made no progress at all in it, I'm just repeatedly wabba-ing my atro to no effect.
EDIT: Nevermind, I took a step outside the box and wabba'd the two guards on the other side with Pelly.