Tatsumaki vs. Psykos & Orochi

The universe has been heading toward this outcome from the very beginning, and I've been chosen to lead the operation.

Tatsumaki vs. Psykos & Orochi is the battle between the S-Class hero Tatsumaki and the leaders of the Monster Association, Psykos and Orochi.

Prelude
Psykos attempts to fight Tatsumaki by herself, but despite her best efforts, she is eventually overpowered. However, Orochi, having survived the battle with Saitama and absorbed enough monsters to regain his strength, fires his energy blasts from multiple directions with his dragons distracting her. Psykos flies down to Orochi's body, confident that if she combines with him her psychic powers will be amplified enough to defeat Tatsumaki. Orochi merges with Psykos, only to reveal that he plans to take control of her instead. After a mental battle, Psykos takes control of the merge and starts to fight Tatsumaki, although Orochi reveals that he still has a level of control at times, as shown when Psykos's face deteriorates into a mass of flesh resembling his own.

Battle
Orochi's dragons fire another round of energy blasts, although Tatsumaki easily kills them. Tatsumaki and the two villains clash, the bolts of energy destroying more of the headquarters. The hero notes how much more difficult maintaining her barrier has become, and the fusion increases the gravity around Tatsumaki. Although the female hero proclaims gravity manipulation will have no effect on her, she feels an increased pressure within her barrier.

From the fused body, a larger, white form of Psykos with long, flowing hair emerges, cupping the hero's barrier in her hands. Psykos explains how she switched the inside of the barrier with the outside, corrupting it. Tatsumaki realizes she can't disable the barrier, and Psykos gleefully remarks that the energy she's pouring into the barrier combined with Tatsumaki's own energy output will keep increasing the internal pressure, but also knows that if she releases the barrier, she'll be crushed by the overwhelming gravity. Psykos declares Tatsumaki's flesh and blood won't go to waste, and the female hero beings to bleed from her head and many different parts of her body due to the tremendous strain on her.

However, Psykos shouts in pain, releasing Tatsumaki from her grasp. She notes that a large portion of cells was suddenly eliminated, having been obliterated by Saitama's punch, and also wonders what the slippery feeling was, which was Saitama's bald head. King informs the other heroes that he's secured Tareo and he's heading towards the surface. Tatsumaki, complimenting King's skills, declares that with the child's safety secured, she no longer needs to hold back, and prepares to fight Psykos and Orochi with her full power.

Tatsumaki then drags the entire Monster Association Headquarters to the surface. She simultaneously creates protective barriers around the entire strike team, except for Flashy Flash (who lost his locator) and Zombieman (whose clothes were destroyed by Homeless Emperor) The barrier around Pig God is broken by Gums, while Iaian breaks his own barrier to rescue the unconscious members of Narinki's Private Squad. King and Tareo are able to reach the surface unharmed, and are found by Genos, who incinerates a large portion of Orochi's mass around them. The three spectate the ongoing battle in the skies. Despite Tatsumaki's attack, Psykos and Orochi emerge from the ruins of the headquarters completely unharmed.

The aftermath of Tatsumaki's base lift can be seen from miles around, and is broadcasted on television. The severity of the damage inspires many previously wounded heroes to head to the scene, including Metal Bat, the Blizzard Group, and the Tank Topper Army. Civilians flee the scene en masse.

Psykos taunts Tatsumaki's power, but the heroine notices that the monster fusion seems to be gaining power from its roots, and targets the pedestal beneath them to sever the connection. The attack fails to work, and Psykos responds in turn with her own beam attack, which cuts into the oceans and severs a large part of the Earth's crust, launching it into space. The severed area slams back into the water, sending massive shockwaves and tsunamis back around the planet. Tatsumaki is shocked by the power display and realizes it could not have been attained naturally. Genos notes that the energy involved is of an undetectable scale and appears poised to engage, but King implores him to leave the fight to Tatsumaki and help flee the scene with Tareo.

Psykos tells Tatsumaki about a "God-like existence" that she witnessed when fusing with Orochi, where she attained divine power and enlightenment. She explains her plan to absorb all life on the planet and become the Earth itself, a plan that Tatsumaki mocks. Growing angry that she is still not being acknowledged by other espers, Psykos and Orochi launch a relentless attack on Tatsumaki. Tatsumaki avoids the attack and confirms the evacuation of the surrounding areas, before lifting a series of massive chunks of earth from the sea, which halts the tsunamis. She launches them at Psykos and Orochi in a barrage; the fusion tries to intercept the rocks with their energy beams, but Tatsumaki is able to hit and engulf them with the remainder. Once again, Psykos and Orochi emerge unharmed, and the fighters engage in a massive psychic battle that creates a gigantic whirlwind in the skies above them.

Tatsumaki uses decoys made of rocks to hide her position from Psykos, and infiltrates the fusion by climbing inside of them from the base of the pedestal. By disguising herself as energy and reducing her output, she is able to discern the position of Psykos' true body, and uses a rending attack that crushes the fusion of Psykos and Orochi from inside their barrier. However, the fusion regenerates from the attack and spears Tatsumaki through her left hand. Before Tatsumaki can counter, she is impaled through the other hand, the Fusion Monster then discharges electric shocks to stun Tatsumaki and make her lose control of her powers, causing Atomic Samurai and Puri-Puri Prisoner, who just arrived on the surface, to fall to the ground due to losing their barriers created by Tatsumaki.

With Tatsumaki now rendered helpless, the disfigured fusion of Psykos and Orochi prepares to finish her off with a massive energy attack. Just as they launch their beam, it is intercepted by a wave of energy from the ground, which launches both beams into the sky.

Trivia

 * Orochi's and Psykos's fused form's initial design is based on a neuron.