Do you see the damage popping up or hear the sword's clashing? I beat the game over a year ago on hard and the boss is easy, but there's a pretty small window for the attacks to come through while he's high so you need to time your swings right while jumping, occasionally though he gets high enough to be out of reach completely.

Did you upgraded your sword? It's important, cause it's the main source of you 'attack' stat. Also - try to attack his "front" with aerial spin attack - just press attack button immediately after jumping. This should work, especially you are at 21 level.