so you want to janus? buy all 3 types of poison potions, then throw them on the ground near him. They'll smash and slowly kill him. I aint tried it yet but it werked with the masses of people in Ars magicana. Drop them one at a time (hold Ctrl when you drop them to choose how many.

"Dont you Feel Guilty after stealing from a hard working, honest merchant?"and the Cutpearse Replied "When i find one to steal from, ill let you know."