Saitama vs. Deep Sea King

Saitama vs. Deep Sea King was the battle between the C-Class hero Saitama and the Deep Sea King, a Demon-level Mysterious Being.

Prelude
Mumen Rider is sent flying by Deep Sea King and is caught by Saitama, who arrives just in time to save him.

Battle
Saitama catches Mumen Rider and congratulates him on a "nice fight", prompting Deep Sea King to ask about the new "weakling" that had appeared. Saitama notices Genos and asks if he is alright, to which the cyborg replies with weakly.

Sighing, Saitama claims that he will pound the "seafreak or whatever it's called", angering Deep Sea King who rushes over and punches Saitama in the back of the head. While his head wobbles, Saitama remains unharmed and unfazed, which impresses Deep Sea King, who proceeds to monologue about how he stands atop all life on Earth. Saitama interrupts him, saying that it is raining and that they should start the fight. Furious, Deep Sea King throws a full-power punch at Saitama, but before it can connect Saitama casually punches Deep Sea King in the stomach, ripping a hole in his torso and killing him instantly. The crowd cheers in triumph and happiness, elated to know that Deep Sea King had finally been defeated and they were safe. Genos smiles at his master's success, while Saitama is rather disappointed with the anti-climactic battle.

Aftermath
This battle had a rather hefty effect on how the people thought of Saitama. Many people thought of Saitama negatively, due to the way he tried to falsely take credit as being the only one who took down Deep Sea King, even though it was true. However, some people still held him in high regard after this battle, such as Mumen Rider, as the two became friends.

Trivia

 * The battle between Saitama and Deep Sea King lasted 8 seconds.