That guy is a pretty shitty fight, imo. I had considerable trouble with him even on Samurai when his mission first unlocked. Farming him for ethereal Austerity set on Nioh was even more unfun.
In the underworld, I got Shibata Katsuie as a helper and what I did was put up every buff I had (carnage, barrier, extraction, cleansing prayer) with weakness on Saito, let Shibata tank him as much as possible and cheesed him from afar with Arc of Chaos spam while hitting him with Mezuki's yokai ability as soon as it came online since it stuns him and deals quite a bit of ki damage. I got him on the first try like this, but it was very close since a single hit from one of his long range attacks pretty much took 80%+ of my hp.
With AoC the fight is somewhat more tolerable, but without a long range attack, in strict close combat unless you find a way to constantly stun him, he's a massive pain in the ass.
In the underworld, I got Shibata Katsuie as a helper and what I did was put up every buff I had (carnage, barrier, extraction, cleansing prayer) with weakness on Saito, let Shibata tank him as much as possible and cheesed him from afar with Arc of Chaos spam while hitting him with Mezuki's yokai ability as soon as it came online since it stuns him and deals quite a bit of ki damage. I got him on the first try like this, but it was very close since a single hit from one of his long range attacks pretty much took 80%+ of my hp.
With AoC the fight is somewhat more tolerable, but without a long range attack, in strict close combat unless you find a way to constantly stun him, he's a massive pain in the ass.