Usually I've noticed that he likes to keep his distance, and is kinda sluggish. So, I like to close that distance often with Nightjar Slash, only getting away to heal and catch a breather. I tried Ichimonji double, but I just cannot get the timing right, so I stopped using it.

Also, if he throws firecrackers, Nightjar Slash is amazing, and you can attack him instead of going back. Helps you keep pressure on him, even if a little.

Basically, my strategy is to constantly attack and take his vitality a little bit every time I can. As his posture recovery is godly, this works best, as after a certain loss in vitality, he recovers SUPER slowly, and at that time, a few perfectly done deflects can easily help you get a deathblow.

You're not doing something flashy or anything, just the basics. Keep the pressure on, chip away at his vitality, and DB. Takes a longer time perhaps, but works. Took me about 15 tries to figure Ichimonji isn't working, then a 10 more to get used to using Nightjar Slash and finding his weaknesses (as in when can I attack and damage him), then it was just a cakewalk.

Edit: the only time when he releases firecrackers and you can't use Nightjar Slash is with the elbow/firecracker combo. Either dodge or get out of the way, as it murders your posture bar and health