To be fair, the difference between an A ranker and an S ranker is still like heaven and earth. Even if he's stronger than an A ranker atm, I do not believe he is at the level of an S ranker. There's too large of a gap and he's somewhere in between. And it's even harder to tell because nobody else uses "levels", so we don't actually know what level an S rank would be.
Might as well be an S Ranker or even more
I don't think he's S rank yet because then the plot would be too boring. I mean from the way this author has been doing things, for him to just so easily become stronger or just as strong as an S level while coincidentally being targeted by one would be too fake/set-up. The most probable conclusion is that the author will find a way for this scary S rank to go away before he exists the dungeon because I'm 99% sure he's not strong enough to fight him yet.
Actually *edit. The author already made him go away in chap. 52.
not yet but stronger than A rank, just waiting for him to reach S rank.
He just did that to an A ranker. Given the a ranker was weakend. BUT WTFUCCKK HE JUST DID THAT TO AN A RANKERR!!