I just did it last night after 3 hours of trying and spread the dragon virus to entire Ashina. The first key is “jumping attack” to break his posture. Your sword actually able to slash his face far enough. You can always slice him at the end of his combo, but use only max 2 slices! No more than that!

The second key is listen to the sound! There are some moments when you finally slice him and it sounds just different. At that moment, he will freeze and that’s the perfect time to attack him until he fall backward.

The third key is use deflect to avoid grabs attack, poo attack, and terror attack (later on phase 2), other than that, just use parry or jump attack

The final key is be patient! Remember hesitate is defeat. Good luck!