[Rataxes]

Memory's rusty, but wouldn't killing the Priestess suffice if you just want the cloak? With the Boots of Speed I'm sure there's some way to dispose of her while keeping out of harms way from the avatar. I imagine the Avatar is way out of your league with the item and spell selection you currently have, so skip, if you can.

On the other hand, the C/R does have a few spells that could work if you do want to kill him. Since melee combat is out of the question with his Imprisonment ability, I'd say your one shot is to hit him from out of range, right? The level 6 spell Fire Seeds is one way, they aren't stopped by MR and hit as +6 weapons, though I doubt you can amass enough to kill him without sleeping, (maybe you can find a way to break away from his hunt and rest in mid-combat?) they do pitiful damage and are mostly useful in combination with Deathblow. Alternatively, if you have some HLA's, a few Implosions should pack enough punch to kill him, it ignores MR, so if you got it, it's a sure shot.