So last night I was helping a friend with his Benediction quest, and noticed an immortal mob called "The Cleaner" spawning, so figured "Hey, lets kite this guy to Undercity!".
So when we got to Undercity and had every guard in the place on him we dropped his aggro (ran from him without hitting so he'd reset to a different target), and some rogues found out you could skill weapons on him while he's busy mauling the guards. Hadn't really thought of it ourselves since we had everything maxed and just wanted him to kill Sylvanas (who refused to fight him, the coward).
Apparantly, he spawns if more than one person is there when doing the Benediction quest, and if you're alliance he could probably raise some hell in Ironforge
There's also a way to spawn him in Un'goro Crater for that matter, but haven't had the time to try yet.
Either way it may be pretty tiresome, but it does work for free weapon skill.