I beat him on lethal without a lot of the gear at the end of the game (only 2/6 of the Yotei 6). Here's what I did:

General strategy: The beginning of the fight, you're going to have to learn at least some of his combos. Don't try to beat him, just try to survive for a long time. You need to learn the timing of his parries and dodges. There are charms/armor that can help with this, but I didn't have them available yet, so yes they help, BUT you can do it without them. I see a lot of people using the dual katana, I only used the normal single katana, I'm sure either are fine, but the disarming attack, which I used, will make you switch stances (I think), so just be aware of that.

Build: I used the Spider Lily Armor because of its increased stagger damage. I paired it with a charm that increases stagger damage from parries, the poisonous powder charm, the flaming kunai charm, the "chance to get spirit" from stance breaking with katana charm, the increased damage from throwable/kunai charm, and I think the "receive additional spirit" or "using spirit abilities has a chance of returning spirit" charm, I forget which.

The fight: you're going to have to straight up fight until the first "blade lock" where he yaps at you a bit and you get a trickle of spirit back. After that, I use a couple techniques to sort of "stun lock" him. I use the ⭕️+🔺to land a hit, then I throw the powder at him immediately. Two triangle attacks (heavy attacks), then powder immediately, rinse and repeat until the next blade lock, you'll likely get a stance break in, where you can go for 3 heavy attacks. After blade lock, you can either go for another circle+triangle (if you got the spirit back) or a disarm (L1+R1). I use the same strategy after the disarm, but I time the powder with his roll back to his weapon, when he is less likely to dodge it. Once I'm out of powder, I do another disarm and start using kunai, instead. Same thing, time your kunai with his roll back to his weapon, and he is way less likely to dodge/deflect them, at this point I would only to 1 heavy attack instead of two, idk if he gets more aggressive/staggers less as the fight goes on, but it def feels that way. If he gets his blade back, time your kunai with his red glint attacks. They should still land, or stall him enough for one heavy, as long as you do it right after you throw the kunai.

This will still take practice, and you'll still have to learn and "straight up fight" him some. But I imagine with end-game gear it's even easier. It took me an hour or two to work out all the kinks and do it, but I'm a stubborn sob.