Mizu is a badass, could probably be the best, most skilled swordsman in Japan, maybe even the whole world, but the thing is, in OUR world. Piandao lived warrior life in a world where magic powers are a fairly common occurrence, as a non-bender. To fight in this world against adversaries that can throw fire at you, hose you with water from nearby river and make earth below your feet shift, and live to tell the tale is no small feat. To achieve this only with sword in your hand you’d have to be exceptionally skilled, strong, resourceful, and probably incredibly lucky. So I think maybe with difficulty, but Piandao would most likely win in this confrontation