Saitama vs. House of Evolution

Saitama vs. House of Evolution was the battle between the amateur hero Saitama and the House of Evolution members Beast King, Kamakyuri, Slugerous, Frog Man and Ground Dragon.

Prelude
After Mosquito Girl is defeated by Saitama, the House of Evolution takes more interest in Saitama, so they send several operatives to bring him in for experimentation. Sometime after, at Saitama's apartment, Genos simply asks Saitama to teach him how to become strong like him. Saitama warns Genos that it's going to be tough and agrees to train him. But at that moment, Genos turns, saying that he senses a presence moving at high speeds towards them, when Kamakyuri smashes through the roof.

Battle
Kamakyuri arrives at Saitama's house first, crashing through the ceiling and immediately has his head punched off for the trouble. Saitama and Genos then go outside to meet the threat where they are confronted with Frog Man and Slugerous. Saitama promptly buries their heads in the ground complaining about how they made a hole in his ceiling. Suddenly, two hands emerge from the ground by Saitama's feet, grabbing his legs and dragging him underground, until only his head protrudes from the sidewalk. Genos begins to rush to Saitama's aid, but is intercepted by Armored Gorilla. While Genos is fighting Armored Gorilla, Saitama is approached by Beast King. Beast King congratulates Ground Dragon on incapacitating Saitama, but notices that the unfazed hero is falling asleep due to how pleasantly temperate he found it underground. Beast King starts to get angry and threatens to scratch Saitama's eyes out, at which point Saitama simply steps free out of the ground. Beast King and Ground Dragon are momentarily stunned while Saitama gives them a chance to apologize for destroying his ceiling. Beast King is undeterred, however, and tells Saitama to prepare himself to witness the true power of the Beast King. Saitama cuts him midway to remove some dirt from his belt buckle. Once he's finished, Beast King repeats his previous statement. Beast King then attacks Saitama, who easily avoids his claws and slicing air currents, which are powerful enough to destroy buildings in the path of the slashes. One of the slashes accidentally kills Frog Man and Slugerous, but Beast King puts it aside, saying that it is just the law of the jungle. Beast King is enraged that none of his attacks are hitting and goes berserk, transforming and breaking his armor, and sends a flurry of attacks at Saitama, which he calls Lion Slash: Meteor Shower. However, Saitama dodges them all before striking back with his Consecutive Normal Punches. The attack obliterates Beast King's entire upper body, prompting a terrified Ground Dragon to run away by digging underground, but Saitama appears in front of him through a hole of his own making and punches Ground Dragon through the earth and up into the sky.

Aftermath
After Saitama kills Beast King and Ground Dragon and comes back to the surface, he witnesses Armored Gorilla dismembered while Genos has his hand extended out in front of the cyborg. Armored Gorilla tells Genos that while he defeated him, he has no chance against the Beast King. Saitama then walks up to Armored Gorilla and shows Beast King's eye in his hand. Armored Gorilla immediately becomes terrified, losing his previous robotic voice and pathetically begging for his life to be spared. He then tells them everything they want to know.

Trivia

 * The battle between Beast King and Saitama lasted 58 seconds.