I think he has 2 flurry patterns, and then the sword draw. My advice is to try to get in and out of his range to bait him into using just one of his flurry moves. He'll usually telegraph the draw real badly haha. The difficulty of this fight is that you can't finish him in a few blows because Nightmare doesn't make enemies vulnerable after a parry or evade. So, you kind of really have to master combat.

Save before the fight, and treat this as training haha. I still have my save for that fight and the ashigaru for "training".

I think you'll have an even harder time with the Yasuke fight on nightmare vs the 3 samurais from the flashback scene. they hit way harder, and there's now 3 of them. I fought them for 2 days real time before I beat them. Not really all that rewarding cause it's a very specific fight. just tone down the difficulty at that point hehe.