Like... the boss is really really fun to fight, but is there a reason why he can combo you from 100-0 in like a second sometimes? It doesn't help that if the camera just so happens to get stuck on a pillar, you can lose sight of him and die in a second.

I feel like for a fight this long, this sort of 100-0 combo shouldn't exist honestly.

Ima be honest bruh I died about 40 times on great shinobi and only once on this one, if he still kept his shinobi charm this would be much harder. Though it was much more fun to fight this one than great shinobi because it's faster paced.

This was hard and tough but it was IMO one of the best boss fights from any Fromsoftware game.

Perfect balance between challenging and fun. Fighting this guy (father version) felt like a Samurai / Ninja dance.

It was great.

Fun fight, significantly harder than his "Great Shinobi" version.

If you came here looking for advice, here is what worked for me as a non-sweat, casual player:

Phase 1: Use a "double ichimonji" in his openings. Especially with the perilous move that he spams a lot and needs to be jumped: deflect the large overhead swing at the end of the combo, and then, immediately after, jump towards him (otherwise you'll miss since the aforementioned deflect will push you back significantly) and do ichimonji.
This will wear his posture down significantly whilst regaining your own. When he does the simultaneous firecracker + swing combo this also works.

Phase 2: Swap ichimonji for whatever your most powerful ability is. In my case, it was "mortal draw".
Assuming you used no prosthetics in phase 1, you should have plenty of spirit emblems since ichimonji is free. Use them all up to wear him down as quickly as possible. Swap back to ichimonji when you're out of emblems.
I find his second phase to be inconsistent because of his constant teleportation and the damn bird blocking your view at the worst possible moments, so I prefer to speed it up.
Talking of the bird, as soon as you see him disappear, look for the bird and run away from it, as that is where he will re-appear and try to smack you.
If you see firecrackers thrown randomly without any combo or sense, he's using them as a smoke screen for disappearance so get ready for him to re-appear and try to hit you.

General: Play aggressive, otherwise his monstrous posture will never go down. However, it's a slow fight even when aggressive, so don't overdo it. He does crazy damage, so trading is a death wish.
Take any mikiri counter that you can get to wear him down faster.
If you can, pin him down against a wall. You can clash swords for quick posture damage as he is really helpless if he can't jump around and run away; But this is a double-edged sword: if he pins you down instead, you're dead in a combo.
People below all mention different prosthetics, but in my case I wasn't able to get much out of any in this fight. You just have to deflect and not make too many mistakes. But I'm bad with prosthetics in general so perhaps check out their advice in that regard instead.

easier than great shiobi and much more fun
change my mind

Owl Father who art thou in heaven

I feel like Phase 2 is way easier than phase 1 - due to his new owl attacks, he's on you a bit less than in phase 1, and one of those owl attacks - the fire owl followed by the long thrust, is relatively slow and very easy to mikiri counter as long as you avoid the fire owl. It means he's doing less firecrackers and sweeps and more thrusts, and Mikiri basically trivialises phase 2 since he does that fire owl + thrust attack fairly frequently, at least 3 times per fight in my experience.

Why would he comment on the prosthetic if it's in the past, when Sekiro didn't have the prosthetic? This doesn't make sense.

Any1 know if you can do Fountainhead and come back to fight him?

I am sorry to say this and pls don't hate me but i did owl on my 1 try but it was pretty close . I think you just gotta have a good timing/reaction time to do him but its fairly easy just jump over his fire owl and mikiri counter his thrust attack after that (Phase 2).
Pls dont hate me for saying this