Saitama vs. Boros

So he's come... This planet's finest warrior. I can sense no limit to his energy. Saitama vs. Boros was the battle between the B-Class hero Saitama and Boros, leader of the Dark Matter Thieves.

Prelude
Soon after defeating Geryuganshoop, Saitama breaks into Boros' throne room. Boros introduces himself as "Dominator of the Universe" and explains that he had attacked A-City because of a prophecy that he would find a worthy opponent on Earth. He also states that he went on a rampage by shattering planets and razing civilizations due to boredom and wanting to find a worthy opponent.

Battle
Saitama throws a swift punch at Boros, disgusted that Boros would go through all of that just to find a worthy opponent. However, the punch does not kill Boros, but instead shatters his restraining armor, which was used to seal his own power. Boros explains that Saitama has broken his armor to which Saitama responds with an "OK". Boros starts to transform into his released form and charges at Saitama.

As they clash, Saitama continuously blocks Boros' attacks. Saitama then finally throws out a punch that takes Boros' arm off. The fight continues to rage on as Boros knocks Saitama around the throne room and they eventually spiral out to the top of the ship. Boros states that Saitama is the only one out of countless beings to survive this long against himself. Boros then starts to release destructive energy from within himself which damages the top of the ship, but is still unable to harm Saitama. The battle starts to reach its peak as Boros begins gloating about his regenerative abilities and grows his arm back. Saitama stops Boros short in his monologue and asks if Boros is done with his gloating yet. Boros responds by unleashing his most powerful transformation yet, "Meteoric Burst".

With a thunderous onslaught, Boros sends Saitama flying with immense speed and continuously follows and assaults Saitama. He is able to land hit after hit and continue to follow a rag-dolled Saitama around the ship so quickly that he starts destroying his ship and killing some of the aliens within. This subsequently leads to Saitama getting kneed all the way to the moon. Saitama still remains unharmed and manages to jump all the way from the moon back to Boros' ship. This causes the ship to start falling to the ground on impact.

Boros attacks Saitama again, but Saitama finally attacks back with a devastating blow to Boros to which he is sent back and dazed. Shortly afterward, Boros is hit with Saitama's Consecutive Normal Punches, which splatters Boros. Using his regenerative abilities, Boros puts himself back together and starts to initiate his final trump card, Collapsing Star Roaring Cannon, which he claims will decimate Saitama and destroy the surface of the planet.

Saitama responds to this by unleashing his own trump card, Serious Series: Serious Punch. This completely nullifies Boros' massive energy attack and the pressure from it alone obliterates Boros. Additionally, the punch parts clouds across the globe for thousands of miles.

Aftermath
Boros lies dying on the ship after Saitama's Serious Punch. He states that Saitama wasn't even using his full power and that the prophecy he had been told was a lie. Shortly afterward, Boros dies and the spaceship starts to fall onto the ruins of A-City where the S-Class heroes are located.

Trivia

 * The battle lasted 5 minutes and 20 seconds.
 * In the anime, the battle presents many similarities with Saitama's dream fight against the Subterraneans in terms of poses and the heartbeat, creating a connection between the first and the last episode of the season. This further highlights how Boros can relate to Saitama's existential crisis and felt the exhilaration of a real fight like Saitama did in his dream.