Heroes vs. Monster Association Executives

From here on out it's a free-for-all. Most amount of S-Class kills gets to lead, how 'bout that?

Heroes vs. Monster Association Executives is the battle between a large number of heroes of the Hero Association and the Council of Swordmasters against the Monster Association executives, Homeless Emperor, Black Sperm, Fuhrer Ugly, Evil Natural Water, and Gums.

Prelude
While Tatsumaki confronts Psykos and Orochi, the other Monster Association executives are locked in their one-sided battles with the heroes. Tatsumaki interrupts each encounter by shielding the heroes with energy barriers and bringing them to the surface, leaving the executives behind. The heroes help Tatsumaki defeat Psykos and Orochi, with the Monster Association base being destroyed in the process.

Presuming their foes all destroyed, the heroes anticipate victory. However, all of the executives are revealed to have survived, and reach the surface immediately after the last battle concludes, taking the heroes by surprise.

Battle
Homeless Emperor arrives on the surface and immediately unleashes a barrage of orbs against Zombieman, taunting the hero's claims about humanity. He derides Orochi and Gyoro Gyoro for their defeat, giving Sweet Mask an opportunity to attack, only for Black Sperm to burst out of the ground and grab the hero mid-air. Fuhrer Ugly arrives immediately afterward and unleashes a Face Crushing Punch on Sweet Mask, before splitting him in two and tossing him away. The three executives agree to compete with each other for the title of Monster King, with the goal being the most S-Class kills. Child Emperor tries to drag Zombieman to safety and alert the other heroes, but is spotted by the monsters.

Fuhrer Ugly steps on Zombieman's regenerating body and taunts Child Emperor while readying a punch, but is interrupted by a massive orb launched by Homeless Emperor. The orb impacts the ground in a massive explosion, with Atomic Samurai, Iaian, Pig God, and Tanktop Master, as well as Fuhrer Ugly, only nearly avoiding it. Atomic Samurai tries to contact Child Emperor, who seemed to have been caught in midst of the explosion, but gets no response, just as Evil Natural Water emerges from the ground. Iaian immediately recognizes the threat and readies his sword in fear. Pig God and Tanktop Master are similarly barely able to avoid being eaten by Gums, who bursts forth from the rubble.

Homeless Emperor, Fuhrer Ugly, and Black Sperm quarrel with one another, but agree to temporarily work together as they head out to confront the rest of the now disoriented and split up heroes. Elsewhere, Genos arrives back on the surface (having been abandoned by Drive Knight) in danger of exploding, but is rescued by Fubuki, who manages to cool his overheating core. Puri-Puri Prisoner sets off to find Child Emperor beneath the rubble, while the rest of the heroes bear witness to the carnage. Homeless Emperor targets the weakened Tatsumaki with a massive barrage of orbs, but is disappointed to learn that his target was yet another of the esper's rock decoys. The real Tatsumaki, waning from her injuries, arrives and insists that others leave the battlefield, ignoring her sister's concerns over her injuries and pleas to help out. Tatsumaki departs alone, but the unswayed Genos and Bang follow after her in support, leaving Bomb behind to watch after Fubuki, and keep an eye out for the still-at-large Garou. Tanktop Master and Pig God struggle against Gums even working as a pair, and are caught off guard when Fuhrer Ugly discovers the unconscious members of Narinki's Private Squad, Bushidrill, and Okamaitachi. Taunting the two heroes, Fuhrer Ugly lifts the barely conscious Sosshi into the air before tearing off his arm. Gums takes advantage of the shock to bite down on Tanktop Master, while Fuhrer Ugly throws a punch to finish off Sosshi. However, his arm freezes before it can crush the mercenary, having been stopped by Tatsumaki. Pig God leaps into Gums, freeing Tanktop Master, and the tank-topped hero helps Pig God collect the unconscious mercenaries and heroes before throwing them to safety. Fuhrer Ugly frees himself from Tatsumaki's psychokinesis and punches the heroine into a rock, while Tanktop Master leaps into the ugmon monster with his Tank Top Tackle. Fuhrer Ugly counters with a Forearm Caving Punch and Tanktop Master is instantly overpowered with the follow up Full Body Caving Punches beating the hero to near death. Gums eats the helpless S-Class hero as Fuhrer Ugly turns his attention to the seemingly lifeless Tatsumaki. Before Fuhrer Ugly can get his hands on Tatsumaki, Bang and Genos interrupts the fight as Bang kicks through the jaws of Gums and rescues Tanktop Master while Genos incinerates Gums' insides. Fuhrer Ugly quickly jumps in front of Bang and launches Face Caving Punch, however Bang is able to counter the punch easily and sends it back at Fuhrer Ugly's face. While the two heroes are checking on Tanktop Master's condition, Gums launches a surprise attack at Genos and entraps him within his jaws. Fuhrer Ugly once again challenges Bang with his brute force, launching Dignity Caving Punch, but once again, the punch was easily countered and sent right into Gums while Bang launches a flurry of attacks at the helpless monster. Genos and Bang finally escapes with Tatsumaki and Tanktop Master, both of whom are heavily injured. Gums becomes enraged by Fuhrer Ugly's misaimed attack and proceeds to swallow him. The two monsters engage in a brief scuffle before Fuhrer Ugly tears Gums in two from the inside out. Covered in Gums' digestive juices, Fuhrer Ugly becomes Vomited Fuhrer Ugly. Appalled by his increased ugliness, the dismayed monster swears revenge on Bang. Meanwhile, Atomic Samurai and Iaian fight to repel Evil Natural Water's jets. Finding regular sword techniques to be useless, Atomic Samurai struggles to think of a method to defeat the mysterious being, while Iaian worries what would happen if the monster reached the ocean. Just then, Iaian's sword is finally destroyed by the jets of water leaving him defenseless, but as another barrage is about to hit the hero, Superalloy Darkshine interposes himself between Iaian and the attack. Darkshine is completely unharmed, and takes on more of Evil Natural Water's attacks, comparing them to a pleasant shower after exercising, before blowing the mysterious being apart with a single punch. While Bang and Genos continue their retreat, Black Sperm catches on and throws several attacks in a wall formation, intent on stopping them. Bang is able to deflect the punches, but Genos takes the brunt of the attack and is nearly sent into a chunk of debris, though he lands safely as a timely Atomic Slash turns the debris to dust. With Evil Natural Water dealt with, Atomic Samurai catches the attention of his "special blade testing monster," who in turn taunts the hero for trying to fight him a second time. This time, however, Atomic Samurai uses the blunted edge of his sword to make incomplete wounds on Black Sperm's body, dealing damage while preventing him from multiplying. More of Black Sperm's clones chase after Bang and Genos, but Tatsumaki promptly regains consciousness and freezes the monster's bodies mid air, giving Darkshine an opportunity to obliterate them. Bang, Tatsumaki, and Darkshine join Atomic Samurai and Iaian (who is now armed with his master's second sword) in staying behind to fight Black Sperm, while Genos rushes Tanktop Master to Fubuki. Darkshine taunts Black Sperm for his lack of "shine," but the monster is only annoyed - even while facing four of the top S-Class heroes at once, Black Sperm's only concern is that they're looking down on him. Black Sperm attempts to overwhelm the heroes with his mass numbers, but is unable to best their coordinated defense. The cells argue with one another over tactics, with some desiring to split to their limit and create a Sperm Tsunami while others wish to merge and combine their strength, as Darkshine's durability is too great for them to damage in their current state. The bickering distracts the monster, giving room for the Council of Swordmasters to arrive and support the heroes. Now facing even more foes, Black Sperm is put under pressure, and he asks Homeless Emperor for a supporting barrage, but the human villain refuses. Homeless Emperor looks on and plots to strike at Black Sperm once he's dealt with the heroes, thinking that "all is as the heavens will it to be." Unable to get any support, Black Sperm pushes harder against the heroes, but Nichirin is able to slow their momentum by destroying a massive section of the landmass. This provides an opportunity for his top disciple, Spring Mustachio, to use his Tomboy Barrage, a move Nichirin says is dangerous in populated areas. The other swordsmen, Amahare and Zanbai, also manage to hold the line. Genos is able to reach Fubuki and delivers the unconscious Tanktop Master to her, before returning to the battlefield. Seeing Tanktop Master's poor state, Fubuki uses her healing technique to give him some of Bomb's power, helping him remain stable. The moment is broken by the arrival of a heavily monsterized Garou, armed with sheer killing intent. Bomb calls out to Bang as he faces down Garou, and Atomic Samurai agrees to cover for the old martial arts master so that he can properly handle his former disciple. A second battle begins. Pig God arrives back on the scene shortly after. Black Sperm is able to score a temporary win when he gets a hold of Tatsumaki, but the heroine is saved by Genos, who incinerates the clones. Black Sperm retaliates by smashing Genos into the ground, though the cyborg quickly recovers thanks to his "contemporary art proof" jets. The two notice a disgusting stench coming from the battlefield, and watch as Vomited Fuhrer Ugly returns to the battle, searching for Bang. Black Sperm taunts Fuhrer Ugly's new appearance, prompting the latter to spit acid on the clones, which rapidly dissolve them. Superalloy Darkshine wastes no time in confronting the monster, launching his Superalloy Double Bazooka, but Fuhrer Ugly returns with his Arm Dissolving Punch and their fists clash. Darkshine recoils his fists as he realizes that the stomach acid is melting his hands to the bone. The monster spits on the hero's chest, sending him down to the ground in terror and pain. Zanbai attempts a Peaceful Death attack with his sword, but Fuhrer Ugly simply catches the blade before spitting acid in the swordsman's face, causing it to melt. Atomic Samurai and Nichirin then angrily launch an attack on Fuhrer Ugly, but even their combined Atomic Slash and Iai Rahu-Asura Slash cannot cut through the vomit, and both of the swordmasters' blades melt in their hands. Fuhrer Ugly follows up on the group of swordsmen with a Full Body Melting Punch, but Nichirin and Amahare are able to push Atomic Samurai, Iaian, and Spring Mustachio out of the way of the attack. The two swordmasters are not as lucky, with Amahare being fully covered and instantaneously killed. Nichirin loses the entire lower half of his body, but is able to remain conscious long enough to hand off the Sun Blade and leadership of the Council of Swordmasters to Atomic Samurai, and give some parting words to his disciple before finally dying. Black Sperm takes stock of the situation, seeing that most of the opposition against him is outright defeated, and the last standing fighters (Genos and Tatsumaki) are being overwhelmed. He turns his attention to Fuhrer Ugly, who continues to taunt Black Sperm and melts more of the spermatozoon's clones with vomit. Black Sperm realizes that the biggest threat to him is not the heroes but the other cadres, and begins to fuse together as Golden Sperm. Golden Sperm blitzes Fuhrer Ugly with a barrage of punches, and when the monster responds with its acid, Golden Sperm merely lets it hit him. As the acid drips off, Golden Sperm is revealed to be unharmed, and he takes Fuhrer Ugly down with a single punch. Before Fuhrer Ugly can recover, Homeless Emperor comes into action and unleashes his Straightline Bombardment into Fuhrer Ugly and blocking the monster's counter acid spit. The combination of powerful attacks leaves Fuhrer Ugly reeling, and the ugmon storms off, vowing to get revenge on Bang, and saving his vengeance against the cadres for a later time. Homeless Emperor and Golden Sperm let the monster leave, presuming he will return and become a slave for them once his spirit is finally broken. Golden Sperm then finds Darkshine reeling on the ground at the loss of his shine and ridicules the hero for his weakness. Seeing a being with even an even bulkier shiny body breaks Darkshine's will, and Golden Sperm sends the man flying with a punch. A group of 10,000 Black Sperm cells dispatched to corner Fuhrer Ugly are surprised by Metal Bat, who interrogates them as to Garou's whereabouts. The Black Sperm clones attack Metal Bat in response and are promptly beaten up with the hero's signature weapon. Metal Bat then laments having killed the targets before getting the info he wanted. Homeless Emperor unleashes his Extreme Carpet Bombing on Atomic Samurai, Iaian, and Spring Mustachio, with the two A-Class disciples forced to protect their masters in the face of an overwhelming volume of attacks. Genos and Tatsumaki are swiftly overwhelmed by the remainder of Black Sperm's cell stock, with Genos losing his limbs in the process, and resorting to dragging Tatsumaki toward safety with his teeth. Revealed to have been buried beneath the rubble and saved by his Underdog Man at the last second, Child Emperor sends a distress signal to Metal Knight pleading for help and prepares for the inevitable loss of oxygen, only to be rescued by Puri-Puri Prisoner's timely arrival. The heroes on the surface seem on the verge of being utterly defeated, and the remaining Monster Association executives taunt their opposition's death throes. Their premature gloating is interrupted by an incredible thumping noise that permeates the battlefield - the King Engine roars. The Strongest Man on Earth stands alone and looks down on the monsters.

Trivia

 * With a total of 24 combatants as of Chapter 151, this fight surpassed Heroes vs. Nyan for the most named characters involved in a battle in the manga. It is also the longest continuous battle in all of One-Punch Man by total number of pages and chapters, being longer than even the combined Tatsumaki vs. Psykos & Orochi & Heroes vs. Psykos and Orochi fights.