Darksouls 2 - (Cheating) Majin Bu as an Invader

We were just playing Dark Souls 2 when suddenly Majin Buu invaded. As i backstabbed him with my greatsword and he did not lose any health, we knew something is going to be horribly wrong... Steam-ID of "Majin Buu": http://steamcommunity.com/profiles/76...