If you are looking for an easy way to kill Petrus just kill a couple of VKDs and get their big backpacks. Two are enough to kill him if used properly. You can place an active mine as petrus is coming towards you and drop to big backpacks on top of it or you can jsut drop the big back packs and remotely detonate them with a wand of fireballs or striking or holy banana. I't been a long time since I've done this, but i sem to recall that sometimes this kills him completely and sometimes you have to do this twice. I'm pretty sure it depends on whether or not his head is severed off in the explosion, disabling his AoLS.
Now how to kill VKDs. My normal method is to throw stuff at them while invisible, Mistresses super enchanted whips are good for this. Something I rarely do is zap them with wands of electricity - zaps won't set off the gunpowder. I like to use mithril or better traps too: set a pile of traps, take your ring off and lure the VKDs over them. It funny. If you take off their limbs or your agi is high enough (Agi 30, hasted, and running) you can wait (hit "s") for them to come towards you (while visible) hit them, move one square back and repeat. Another funny thing to do is pray to Scabies, GC6 is one of the best levels for this (there is a better level just before Oree's Lair). Then just run around invisible killing necromancers mistresses, orcs, dark knights - training stats and watch everyone's limbs drop like leaves in autumn.
If you have cleaned out all the stores then you can give yourself perma-invisibility my zapping yourself with the numerous wands of invisibility. I started saving up my haste wands for this too. Perma-invis perma-haste, perma-telecontrol (get hungry and eat 10-15 blinkdogs) kicks ass!