The Duel.

While Kumaou's doppelganger began to tire him out, the old man could see the changes in his fighting style.

Where they were previously evenly matched, slowly Kumaou began to predict his doppelganger's moves and began to counter them. It became easier once he adapted to the fact that he was fighting against his own psyche.

And it happened suddenly when his doppelganger threw a punch at him from his blind spot, but he dodged it flawlessly before kneeing it in the groin. 

His doppelganger was forced to crouch as he continued his onslaught, throwing one punch after another.

It didn't take long for his doppelganger to fall without a hint of life in him. 

As proof of his victory, light particles enveloped the doppelganger and they were absorbed back into the mirror.