Rokudou's been taking punches throughout the entire series, so he's gotten the endurance.
The guy fought against his former master and Ranna.
Rokudou knows his moves and started the fight at full power.
Despite being less experienced in combat than his opponent, he does have a few advantages over him, and that he used well.
Glad he's finally doing the fight this time around, though, this is the type of character growth I like to see!