@Ribath Yeah, sadly I only see this ending with the Oni God leaving for some reason (most likely boredom, interruption from a third party, or he wants to mess around), and MC and others are nearly dead and such. That, or MC uses some secret technique which has to be established right now in order to even have some legitimacy (which it wont) as to why he was able to win (i.e. lazy writing / could not think of a good way to end the fight so bullshitted something in to solve it).
I seriously hope it is neither these 2, contains no NTR, mind control, annoying dramatic bullshit and just ends with something completely from out of nowhere. Like another hero shows up to help out from somewhere or MC just eats him with his skill.