Tareo is running in terror from Black Sperm, who found the child and is chasing and scaring him for fun, when the monster senses an attack coming for him, and dodges the slash. That slash is revealed to be from Atomic Samurai, who ran into them while roaming through the Monster Association Headquarters. The swordsman compliments Black Sperm's nimbleness, and their battle begins as Tareo safely runs away.
Atomic Samurai introduces himself as the hard boiled yet kind-hearted S-Class hero. Black Sperm recognizes this name, although he is not impressed by the hero's appearance. Nevertheless, he says that he has been waiting for Atomic Samurai for a long time, only for the hero to retort that he has no interest in Black Sperm. The monster feels enraged when he senses that Atomic Samurai is looking down on him, and transforms his body into a very large and muscular arm. Black Sperm asks Atomic Samurai if he is mocking him, which the hero confirms, going so far as to say this will be an easy win for him.
Atomic Samurai unsheathes his sword and slices the monster's arm off, explaining to Black Sperm that he has fought countless battles against enlarging monsters and that he is used to it. To his surprise, the sliced arm transforms into another Black Sperm. Atomic Samurai notes that the replicating monsters such as Black Sperm are the most annoying type. Black Sperm mocks the hero's nonchalant attitude, deriding his idiocy while knowing that Black Sperm can simply multiply whenever he pleases, and that no matter how many times he splits, twists or tears him apart, he will keep on multiplying.
While he is talking, Black Sperm's head multiplies into several heads that expand sideways, and Atomic Samurai slices all of these heads in half. Atomic Samurai calls Black Sperm the true idiot, and believes all he has to do to win is shower Black Sperm and his copies with a countless number of slashes. Black Sperm goads the hero into giving it everything he's got, prompting the hero to slash Black Sperm into several hundred pieces. Unfortunately, all the chunks of flesh morph into more Black Sperms, which Atomic Samurai slashes again, and again, the chunks of flesh form into new Black Sperms.
The hero decides to unleash his Atomic Slash, slicing all of the Black Sperm copies into little pieces for a third time. All of the Black Sperm chunks combine into a large black wall with faces of Black Sperm surrounding Atomic Samurai, with the monster claiming that the battle is over, and that this will be his deathbed.
One of the Black Sperm clones punches Atomic Samurai in the chest, causing the hero to cough up small traces of blood. Atomic Samurai is shocked that even after splitting so many times, Black Sperm is still so powerful, lamenting that he has miscalculated the monster's strength. He attempts another Atomic Slash, but the Black Sperm copies assault him with several more punches, causing Atomic Samurai to cough up even more blood.
To try and escape the Black Sperm army, Atomic Samurai cuts the floor below him and falls down several floors. Several of the Black Sperm copies jump down after him, but suddenly, one of the Black Sperms gets annihilated before it can replicate, shocking the other Black Sperms. Atomic Samurai notes to himself that after his battle with Melzargard, he has been thinking about how ineffective his blade was against him, and so he created a technique to increase the speed and power of his slashes in exchange for reducing the range of his attack. He then says to the Black Sperm copies that concentrating his slashes at one point was something especially meant for monsters like him, and with a dark tone, tells the Black Sperms to get reduced to dust. He unleashes his Focused Atomic Slash, killing all of the Black Sperm copies who came down after him.
The hero falls to the ground and moans in exhaustion. He states that Black Sperm was indeed a very tough monster, and also notes that his wrist is hurting more than it has in years. He then remembers he has to save the child, and jumps back up to where he fell. However, when he reaches the top floor, he is greeted with a very large Black Sperm copy with three tiny legs and a monstrous arm. Black Sperm explains that he hid his true self from the beginning and sacrificed a small portion of himself, which really helped.
Atomic Samurai attempts to use his Focused Atomic Slash again, but it is too late, and Black Sperm punches the swordsman to the ground. While Atomic Samurai was able to slash Black Sperm into several pieces, he was unable to complete his Focused Atomic Slash and only ended up creating more Black Sperm copies.
With another army of unharmed Black Sperms surrounding a badly bruised Atomic Samurai, the monsters tell him to accept defeat. For someone like Atomic Samurai, Black Sperm says that he is the worst possible matchup, also adding that Gyoro Gyoro put some thought into this matchup. Despite being badly beaten, the swordsman is not completely demoralized, and he slashes the roof, telling the Black Sperm copies to get buried.
Black Sperm overpowers Atomic Samurai, mocking the hero for underestimating Black Sperm's abilities.