I am a genius
I don't recall ever seeing this method posted before but this works well.
How many times have you been fighting a golem or dark knight or any random humanoid and had it kick you after you severed both its arms off?? Well this happened and I thought "Lightbulb!" had a rookie dark night follow me into GC 3 shop and he kicked out a nice pile of gear for me. The problem was its hard to get gear out the door, luckily the shop had two doors. Gear goes right by guards when the monster kicks and they don't get mad.
Anyway, I had another lightbulb go off. Sulphuric acid!!! Drop it where you want an exit and the monster kicks it breaking the wall allowing stuff to go flying out without causing alarm.
So I tamed the knight brought him to Attnam. Kicked him and used the sulphuric acid trick to empty the shop and get gear out of treasure room and armory.
And then... Lightbulb!!! dropped large mines and RDK kicked them three paces. Go to Cathedral and dropped mines 3 paces in front of Petrus and Taadaa!!! Perfect mine placement under Petrus' chair
Tamed a veteran kamikaze dwarf and had him follow me to Petrus. Kicked him with him directly in front of Petrus and me on the other side and he set off his big backpack and the mines. I died in the explosion but I am certain this is a legit way to peacefully assassinate Petrus with more fire resistance and HP.
Edit: or of course I could have had a second pet standing by Petrus for the kamikaze dwarf to attack and detonate the bomb under Petrus' chair. Damn why didn't I think of that.
Although if I don't kill Petrus directly I probably can't become the Avatar of Chaos so if I really wanted the win I would have to set off the explosion personally. It's still a nice trick for planting a bomb under his chair "Oh and by the way Petrus......KABOOOM!!!!!!!!!!!
Lame other tip:
Same game same level I found an eddy in the space-time continuum and got him to follow me into the shop by taking my ring of invisibility on an off.