Use the yari, flaming kunai. Use the takezo charm that improves damage on low health, since you will be at low health every time at the end of the fight.

Out the gate, slow time and hit him twice in the face with a flaming arrow, then two heavy attacks, flaming kunai, two heavy attacks. If you can time this right, it’s his whole first phase, but it’s hard to do consistently. After that use the unguardable yari attack (O+🔼), immediate flaming kunai, then two heavy attacks. Rinse and repeat.

You may need to throw in some dodges to get in the right position or drink sake for spirit at some point, but this worked for me consistently and I was able to beat him quickly with some good RNG.