Solution: have the other 2 join the fight.
He'll probably copy their weaker fighting power because he can't turn off his copy giving him the upper hand.
Dude has a fucking Sharingan. I really hate characters like that who can perfectly perform moves by seeing them just once. Making training meaningless is the worst trope ever. Worst part is that it's some nobody character that I barely even remember.
Honestly, I've loved this series but this arc has been hot garbage. Slow moving, no development, just psycho bad guys acting like psycho bad guys in order to lead up to Ranna going evil and murdering everyone.
And after looking at the raws:
It gets worse, Rokudou is basically useless and has to be saved by a Deus ex Machina. Dropping it for now. Probably going to drag on for another 2 years before it's over.