Lol, I'm writing this on a reread in 2026, but who cares. Man, readers can be full-on r*tards sometimes, lol.
Rokudou won against Tateshime because Ling Lan basically took 80% of his HP beforehand. Then Ranna took an additional 10% or more with her attacks on him before she got pacified. Rokudou only had to fight against a Tateshima that was basically ready to fall onto the ground himself.
You guys forget that Rokudou only learned kung-fu for literally only a month, as stated by Tateshima. Sure, plot armor won him the fight — but it's still realistic that Rokudou can best a dude that was already plenty beaten and bruised himself.
Try learning some martial arts yourselves. I took boxing classes once myself, for about a month and a half — back in high school. And I can tell you that you don't instantly become Mike Tyson or Ali.
Rokudou hasn't had kung-fu imprinted in his body deep enough, for long enough, that he can reflexively react well to attacks.
Against Yagen — like with Tateshima, Rokudou was also fighting him after Yagen had already taken some damage. Okamori is no slouch, he got a few hits in against Yagen.
Besides, Rokudou's strength has never ever been emphasized by the author as physical. His strength has always been mental, emotional. He wins because he doesn't fall down.
Hell, the author went out of his way to portray that very point with the results of both the Tateshima and Yagen fights. Both times, Rokudou would always be on the ground or on his knees — and the other two would be standing up, seemingly victorious over him. But then, his opponents would fall — and Rokudou would get back up, struggling. In pain, yes, but he would still get up.
Rokudou has very very little self-defense abilities. A month of kung-fu won't turn him into Mr. Badass out of nowhere. His shining trait is still his determination.