Do you remember, the time when the people at the hero association start noticing him as one of the heroes who always finishes his opponent with one punch, hence the system of giving the nicknames that is there in the hero association is where the name One Punch Man is tossed for the first time.

And yeah regarding Madara he was only been able to kill was by Kaguya who betrayed him by stabbing from the back, I hope I remember XD. Otherwise no one was able to take him down not sasuke , not naruto, not guy sensei, no one.