Genos vs. G4

Genos vs. G4 was the battle between the S-Class hero Genos and the Machine God G4 of The Organization.

Prelude
Saitama and Genos are out grocery shopping, when they notice the S-Class hero King. A robot named G4, a Machine God created by The Organization appears and challenges King, however, King asks to use the restroom before fighting, claiming that when nature calls, he can only fight at half strength. G4 gives King 10 minutes, and declares that for every one minute King is late, he will kill one person. It is then revealed that King is not strong, and instead ran away to hide. Although he feels guilty for his actions, he is too scared to come back. Due to King's absence, Genos ends up engaging in battle against G4.

Battle
Genos breaks G4's sword, then initiates a Rocket Punch, sending the robot flying. G4 manages to get back up with the use of his back thrusters, and proceeds to attack Genos with a downwards kick. Genos manages to block the attack, but has taken minor damage to his face. Saitama asks Genos if he needs help, but Genos refuses, stating that defeating G4 would be the perfect opportunity to reach the S-Class' top ten. Genos then breaks through the foot of G4, and continues his battle, while Saitama leaves the scene. Genos eventually uses Incinerate to melt the internal structures of the robot, claiming that its big body was its downfall. Moments later, the large body of G4 completely melts to reveal the true form of the Machine God. Although much smaller now, G4 continues his battle against Genos by unleashing a barrage of laser beams, one of which cuts off Genos' right arm. In order to counter the lasers, Genos hides inside an apartment and comes up with a strategy. In an attempt to force Genos out of the apartment, G4 unleashes a barrage of laser beams, blowing up the entire area. After evading the attack, Genos throws a fire extinguisher and unseals the end of a nearby pipe, which produces a large amount of steam. He then restrains G4 with a cable from his severed right arm. G4 attempts to fire its lasers but finds it impossible due to the moisture dispersing the light. Genos announces that he is stronger than G4, and engages in hand-to-hand combat with the robot.

It is later revealed that Genos has won the fight, but has sustained severe damage to his face and body.

Aftermath
Genos goes to Dr. Kuseno's laboratory and gives him the remains of G4. He tells Dr. Kuseno to use the remains on himself in order to become stronger. The upgrades he gains from G4 prove to be very helpful, as the upgrades assist him with defeating many Demon-level threats he faces later on.

Trivia

 * The battle between Genos and G4 lasted 15 minutes and 34 seconds.
 * This is the first fight of the anime's second season.