MC can shout "I can level up and grow stronger!" at the main Seoul's square.
He is S-rank for god sake and can do as he pleases.
Now some other S-ranks can beat him if go together but until the next couple of dungeons (and some powerful minions).
And then he will be a god-like being and no one can harm him. Yes, they can harm his sister and/or friends, but this is just elaborate suicide.
American S-rank just funny idiot and Darwin award nominee.