So how'd he deal with tanking? Unfortunately, it seems like the whole thing is based on a pathing exploit-- the player can jump up on that ledge at the end of the fence, but the Horseman can't, so he basically "juggles" the bad guy back and forth while dotting and nuking him. The job gets harder when the pumpkins show up, but it's the same thing-- jump up and then jump back down when the Horseman gets close. If Blizzard had known about this, they probably could have fixed that pretty easily.
But I don't mean to downplay Deadmasters' achievement-- clearly he's geared up right for the fight (look at those Affliction DoTs go!), and by the outtakes at the end, you can see that juggling the mobs just right was no easy task. It'll be interesting to see if Blizzard makes changes in the encounter next year, though-- by then, we may be level 80, and who knows what horrors will lie in the SM GY?
[ via incgamers ]