If the old man cannot dodge or defend against the big dude's attacks, then why not go for broke and hope for a double knockout?
Kou said the old man just needs to land one "clean hit"; sure that sounds impossible, but the old man is 1. unpredictable (he won't react the way a professional fighter would), 2. underestimated (despite his age, he has a high tolerance for pain), and 3. thinks on his feet (he helped defeat the Gatekeeper). All I'm saying is, MAYBE there's a chance the old man creates an opening, enough to satisfy Kou and enough for us to feel like he won without "plot armor."