|Saitama vs. Boros|
Dark Matter Thieves' spaceship
|Outcome||Saitama is victorious|
|Hero Saitama||Villain Boros|
|Saitama is uninjured||Boros is killed|
|Previous||Saitama vs. Groribas and Geryuganshoop|
|Next||Saitama vs. Pluton|
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 had also stated that he went on an rampage by shattering planets and razing civilizations due to boredom and wanting to find a worthy opponent.
Saitama throws a swift punch at Boros, disgusted that Boros would go through all of that just to find a worthy opponent. The punch did not kill Boros, but instead shattered his 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 transform into his released form and starts to charge at Saitama. As they clash, Saitama continuously blocks Boros' attacks. Saitama then finally throws out a punch that will take Boros' arm off. The fight continues to rage on and Boros knocks around Saitama in the throne room which will 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. He then starts to release destructive energy from within himself which damage the top of the ship, but is still unable to harm Saitama. The battle will start to reach its peak as Boros starts gloating about his regenerative abilities and grows his arm back. Saitama stops Boros short in his dialogue and asks if Boros is done with his gloating yet. Boros responds by unleashing his most powerful transformation, "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 ragdolled 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 kicked all the way to the moon. Saitama still remains unharmed and manages to jump all the way from the moon to Boros' ship. This causes the ship to fall to the ground from the impact, but not destroy it yet.
Boros attacks to Saitama again, but Saitama finally attacks back with a devastating blow to Boros to which he is sent back and dazed. Shortly afterwards, 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 planet's surface layer. Saitama responds to this by unleashing his trump card, "Killer Move: Serious Series, Serious Punch." This would completely nullify Boros' massive energy attack and the pressure from it alone would obliterate Boros. Additionally, the punch parted cloud cover across the globe for thousands of miles.
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 afterwards, Boros dies and the spaceship starts to fall onto the ruins of A-City where the S-Class heroes are located.