Bloodstained: Ritual of the Night

Zangetsu can be found in the Oriental Sorcery Lab (above the Entrance).

You can cheese this fight easily. I was level 35 and he still made over 150 damage with one hit. I had serious trouble at first because of his high attack damage.

My setup:
- Greatsword (my strongest had 43 attack)
- Greatsword expertise shard
- Equipment for extra damage (power silver ring and the map one - both add strength). Similar equipment will work

Technique to beat him:
Simple. Invert as soon as the fight starts.
That way, most of his attacks will miss. I used a greatsword because of the overhead attack. Any weapon will do but with a greatsword you won't need to use the double jump and you can attack more often and faster (thanks to the shard).
He may hit you with his chain and pull you towards him. Invert again before he can hit you and keep on attacking him.

tl;dr invert

You get this the second time you fight Zangetsu. It's not a difficult fight but he can kill you very quickly if you're not careful. Obvious spoilers in the video.