You could try Batman?'s approach and make a giant pile of wands and explosives then lure Petrus onto it to kill him in one shot, but I'm not sure if you have enough left at this point.
thanks for the attribution but someone posted this approach in the forums ages ago, i just resurrected it.
I think its achievable if you find an early ring of poly control andsource of polyitis- you can just keep polying into stronger monsters until you are something beastly. if you need to become something more powerful just poly into something smart first as a bridge.
It is of course item dependent. you need some source of poly control early on.
I was resting on GC 4 (after accidently kicking in the door of Merka shop Ruh-Roh) and the game randomly asked me if I wanted to turn into a werewolf. I had not eaten a werewolf corpses, and I checked myself wit ha stethescope and i did not have Lycanthropy.