Saitama vs. Vaccine Man

Saitama vs. Vaccine Man was the battle between the amateur hero Saitama and the Dragon-level monster Vaccine Man.

Prelude
Vaccine Man appears and attacks A-City, using his energy balls to destroy buildings. The Hero Association sends Smile Man and Lightning Max to defeat the creature, but the two are easily defeated. Saitama sees the news report and heads out to fight the monster.

Battle
Vaccine Man attempts to kill a young girl, but Saitama saves her. Vaccine Man asks for Saitama to identify himself and he does, saying that he's just an average man serving as an average hero. Vaccine Man is angered by his lack of seriousness and aggressively explains his monster origins. As he talks, he transforms, growing larger in stature and more imposing. As Vaccine Man is about to attack, Saitama kills him with a single punch.

Aftermath
After he defeats Vaccine Man, Saitama is seen pounding his fist in anger because of the lack of challenge the monster gave him. Credit for defeating Vaccine Man went to King, who happened to be near the fight at the same time.

Trivia

 * The "battle" lasted 21 seconds.