King and Saitama vs. Elder Centipede

He just said "understood"... along with the roar of his King Engine.

King and Saitama vs. Elder Centipede was the battle between the the heroes King and Saitama of the Hero Association and the Monster Association executive Elder Centipede.

Prelude
Elder Centipede is charging towards Bang, who prepares to go all out against the monster, when King distracts the centipede by yelling over a megaphone.

Battle
Elder Centipede's attention is diverted from the other S-Class heroes, and he looks down at King. King goes on to say that he brought Blast, the very hero that defeated him many years ago, with him. He tells Elder Centipede to follow him if if he wants to fight Blast again. King then goes on to taunt Elder Centipede with various insults.

King's thoughts are racing inside his mind, as he knows he's giving Saitama a very limited window of time to destroy Elder Centipede and prevent him from burrowing underground and retreating. Elder Centipede then crawls on to the ground and rushes at King, who maintains his ground. As Elder Centipede is just feet away from King, Saitama comes in from behind. King jumps out of the way, and Saitama performs his Serious Series, Serious Punch.

Unlike previous attacks, the force of Saitama's punch is too much for Elder Centipede's regeneration to keep up with, and the monster's body quickly crumbles, marking his permanent defeat.

Aftermath
After Elder Centipede's body is decimated, Saitama notices Genos from far away and waves hello to him. King and Saitama then walk towards Genos and the two martial artists, with Saitama beginning a conversation. He says that he actually feels rather refreshed after defeating Elder Centipede, attributing the stress to playing video games against King. King is skeptical of this explanation. When Saitama meets up with Genos, the cyborg asks him what it is he's still lacking, and Saitama replies by saying he simply needs more power. Even though it's a very obvious statement, Genos is awed by it and begins writing in his memo, much to King's horror. Genos then remarks he will strive toward the symbol of strength that Saitama is.

Omake
In an extra included with volume 17 of the manga that covers the immediate aftermath of the battle, it's revealed that King soiled himself when Elder Centipede rushed at him. He passes this off by blaming the monster's remains for being the cause of the smell. He then asks to visit a pants shop on their way back to Saitama's place, as he 'kinda wants to buy some new pants'.

Trivia

 * This is the last fight of the anime's second season.