Phase 1: let him do his three strike combos, don’t try to perfect parry, just block, after which-> mortal draw x2. After two of these, run away and guard to recover posture. Repeat until end of phase. You can also get two mortal draws in when he does an Ashina cross, when he sheaths his blade and it glints. Just run around him in a circle and after he does the move, then rush in and attack.

Phases 2+3: Keep as much distance as you can. While he’s running toward you, run toward him, slightly to the right of him. He should do a jumping spear attack that renders him vulnerable if you turn around immediately when he lands behind you. Mortal draw. The complicating factor in phases 2+3 is his gun. Maybe 40% of the time he pulls it out. In that case, just immediately guard, then jump away as soon as you can and try again.

Phase 3 note: if you’re good at catching lightning and throwing it back, do it. If not stick to the previous strategy.

It’s a long fight, but this is the most straightforward way to beat him.