@Davy0 Against a Rikishi, your best bet is to take advantage of the rules, sumo wrestlers can't touch the ground, so submission is an option. Striking is too, but you'd be better off using a sniping style of striking with fast jabs, and precise kicks. A sumo wrestler will catch you off the initial push so you'll want to move out of the way as quick as possible.
@subliminal You're right, in that punch wouldn't have knocked out a sumo wrestler but look at Naruse's positioning. He's in close enough range for an elbow, or a knee. Or if he really wanted to hurt him, his testicles. In essence, this is a stupid manga and we should enjoy it for such, if you want realism, read Holyland.