Sekiro: Inner Isshin version 3 (Demon Bell, no Kuro's Charm, no damage)

Another version because it turns out there is actually a way to counter his instant perilous attack in phase 1. Basically you want to be less aggressive and only attack until Isshin blocks (instead of a deflect), a bit like you would do in spear phase, but doing this makes his moveset much more dangerous as it makes him do several much faster counterattacks, as well as a very delayed counterattack to throw you off, a lot like Lady Maria's first phase.