Google headless horseman solo and there are some lock videos from last year that depend on a pathing exploit of jumping on / off ledges / over fences.

From my experience I think a badge geared paladin may be able to solo this, I dont think it's possible by any other class without exploiting pathing. I was going to video my own kills; but honestly it's not that exciting. Two decently geared players (any tank spec + any form of heals) can easily duo this with the new talents (tank abillity to hold all the adds if the dps is low).

Edit: I dunno, maybe with insane DPS and kiting a lock could solo him without exploits ... if you could stay away from him long enough during the body phases to not get owned.