Wtf is happening with me. Currently on ng+3 and looking for Shura ending but I have died to this old bastard more than geni or sword Saint on first try. Hell I didn't even died to a single boss in ng+1 and 2 other than enemy mobs. Even Inner isshin and inner geni didn't took me more than 3 attempts. Wtf is going on is it just me or the second phase is wiping the floor with someone else. Emma sucked on first two tries
but her only bs move was that modified ashina cross which has no indication of deflection timing(like the second blink in case of normal ashina cross) but I use umbrella and it was fine overall. Isshin phase 1 is literally sword Saint phase 1 pretty easy I have no hits on that but phase 2 man is it bs or what? Ichimonji followed by a fire slash and one mind is just wiping me down. I will win it soon I know but I feel like **** dying so many times without knowing the proper way to counter these piercing attacks.

Just finished the boss fight with Isshin Ashina in NG+.
Emma was pretty straightforward, just basically deflect most of what she throws at you and you can posture break her after like a minute or two.
Isshin phase 1 was a bit trickier, since the timing of his attacks is very different compared to other bosses in the game. But once i got the timing down i could perfect parry most of what he does.
Isshin phase 2 took the most time. It felt similarly as annoying as the Demon of Hatred, because it felt like i could only block or parry like 50% of his attacks. For all the big fire attacks I had to run around pretty much all of them, because blocking or even slightly mis-timing the dodge meant that i'd get hit with a burn status. The firecracker trick helped a lot to avoid his most damaging fire attack.
Overall, I agree with some of the other commenters here that phase 2 of this boss is harder than sword saint or owl (father).

The shinobi axe does great damage to him on the first playthrough, I couldn't have beat him without it. After deflecting Ichimonji is a good time to hit him with the full axe combo, as well as at the start of each phase. In phase 2 you can hit him with the axe after dodging his fire blast or dodging the hit after his new flaming Ichimonji.

You can dodge his One Mind attack reliably by finding a safe zone in the fire, waiting for him to approach (the attack never strikes early no matter how near or far you are from his starting point), then dodging forward and left behind him just as the fire dies down. Keep a distance behind him, as the invisible sword slashes actually cover a small area behind him, and stay away from his sides because his follow-through has wide range and decent tracking. THEN you can punish him with an axe to the skull.

the second phase theme is king of a mix of demon of hatred and sword saint, i like it

He's hard and his fire spam is truly stressful, but at least he doesn't have his trusty flintglock pistol

Using the mortal blade as much as possible got me through this

possibly my favorite boss

This is my favorite boss in the game. The music is awesome, the first phase eases you into the fight, then you start the hard part after the first phase. He's fast, fun, and challenging.

Coming from Sword Saint Isshin this fight feels a bit underwhelming