Yeah, this is dumb. I got the boss down to 25% a couple of tries but then choked and quit. I've probably died like 40 times to this samurai dude now.
I feel like there's skill involved but also a lot of luck since you depend on what attacks he decides to use and whether he wants to chain them. Sometimes he'll fight fair. Sometimes he'll combo after a stun attack and that's it, you're fucked, maybe five seconds into the battle.
The stupid charged slash drains almost all of your stamina if blocked, so unless you're on full stamina when he does it, it's a tradeoff between getting 70% of your HP gone in one hit, or getting stunned so he can follow with a 70% riposte. And sometimes he does it twice in a row, because fuck you.
I know, I know, "here's a video of a guy killing him while naked and playing with his toes." I don't care. Just because someone is extra good at hard games doesn't make it less hard, that just means that person is crazy skilled. This fight is just stupid hard for the sake of being hard.