In the battle between Noumu and All Might, All Might says that if Noumu is made to fight him at 100%, he will have to punch him with over 100% of his strenght to defeat him, and that if Noumu's quirk is shock absortion and not regeneration, he might have a limit to how much beating he can take. After he sent Noumu flying, he said in his prime he would have needed 5 punches to defeat him, but he needed over 300. But what sent Noumu flying was only one punch, his "plus ultra" punch. Then, why didnt he just punched Noumu with his "plus ultra" punch from the begining? What were all the other punches for?
I can think of a few reasons.
- Its a Shonen Anime. The supreme rule in Shonen is Rule of Cool. Other than this, tropes like Special Attacks, Limit Break, Finishing Move among others also apply.
- As the flagship fight of the season 1, it was required to be longer than "1-Punch". (Though One Punch Man shows it can work. Lol)
In-Universe Since it was the battle between almost equal opponents, the 300 punches were required because,
- To put Nomu off balance so he couldn't defend against the final "Plus Ultra" Punch. If it were the first punch, Nomu may have successfully defended against it. If you look closely to the battle you'll see Nomu starts off matching All might punch for punch, than his frequency decrease as All might gets more hits and finally it becomes one sided as All Might pummels him around.
- Since All Might knew that Nomu is not nullifying the damage and regenerating it, the speedy punches gradually chipped away his strength until it was down enough to finish in one blow.
All Might new he had a fixed amount of time until his "Super" form lasted, hence to make it count he went head to head with Nomu right until the last moment releasing all his power in the last punch.
If you go and read the manga (and hopefully in the current airing season) you'll see he does something similar
When he goes against All for one, he uses the last embers of One for All in his body to deliver the killing blow. Does that mean the previous punches were meaningless? I think not!
When All Might discovered that Nomu's Quirk was Shock Absorption and Super Regeneration, All Might quickly figured out that he must have some upper limit to his Shock Absorption abilities.
Thus he punched him so many times so that Nomu would've been overwhelmed by the sheer number of punches (and the shock from them) his body had to handle.