One of the hardest bosses in Dark Souls III is Pontiff Sulyvahn, and it could be argued that this is the turning point for the game. The game might be considered relatively forgiving up until this point, where it throws a brutal boss at you like it is nothing.
Despite the difficulty of this boss, one player on YouTube who goes by the name of neptunusequester has managed to issue it a deadly one-hit KO. How? The general consensus is that you need to be on New Game+, and have some pretty high-level gear. Using the ever satisfying riposte mechanic, and a series of buffs to his weapon, neptunusequester had no trouble taking him down.
You can check out the video where he pulls off the deed here.