Beast

The toughest encounter I ever ... encountered ... only played Primordial Hydras, one each turn from the 3rd turn onwards. It took me forever to beat it, and then only by a hair's breadth using a deck with a spammable mechanic for dealing damage to the opposite player. In comparison I beat the final boss in my first go with the green deck I'd been using throughout the rest of the game - legions of huge creatures and exorbitant amounts of mana FTW.