I really had fun with the boss.
Super hard at first but he has quite some weaknesses.
If you can attack his head, with a jumping attack or something then he loses huge amounts of ki and staggers.
Paralysis also very useful to grant you some breathing space. Umbrella or nureonna soul attacks are great for this. Quick cast, cheap and payalsis

I think that if he did his charge attack less frequently and it was a burst attack, people would find this boss a lot easier