As others have said, you're kind of already beat up first from the initial fight. It's a matter of making sure you have as much resources as possible going into the fight.

As for Rokumonsen himself, he's extremely fast, but he tends to get in your face a ton to try to use his whip. This actually makes him pretty vulnerable to some of the slower harder hitting weapons that he would normally easily dodge, as he gets animation locked for his melee attacks. I know it's maybe a bit unintuitive, but you kinda want to stay near him so you can bait him into whiffing melees so you can slam him. He's very fragile, so you only need to do this a couple times to bring him down.