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 monster.

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

Battle
Saitama catches the fallen Mumen Rider and congratulates him on a "nice fight", prompting the 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", angering the Deep Sea King who rushes over and punched Saitama in the back of the head. While his head wobbles, Saitama remains unharmed and unfazed, which impresses the Deep Sea King, who proceeds to monolog 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 the Deep Sea King had 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, and the two became close acquaintances if not friends.

Trivia

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