Before we begin, let's get some ground rules laid for this shall we?
1: I know full well that taking Saitama out of One Punch Man goes entirely against the point of the anime and what it stands for, so no need to tell me so. Blame the writers for making such an interesting universe/set of heroes/ set of villains.
2: I'm gonna have to fudge it a bit. We all know that S class has a habit of just not showing up, even when the threat is clearly dragon or higher.
No one in S class arrived to handle Vaccine Man or Biceps King even when they'd proven to be Dragon level threats, same for the meteor. Even against the Seaking, Puri-Puri Prisoner was only there by coincidence.
In this, I'll try my best to give reasonable explanations for why certain people are in certain places, but sometimes we're just gonna have to assume that the S class heroes here are just less dismissive in general.
3: Spoilers ahead. Not necessarily for things that happened in the manga, as this will, at least currently, only deal with villains from season one, as it's all I've seen of One Punch Man.
No, what may be spoiled here are the powers of the S class heroes that were not shown in One Punch Man season one. If you'd rather not be spoiled on this, feel free to avoid this.
4: I'm gonna try to get as many heroes from every class as I can in here, which means no spamming Tornado. I would like to know who you think should go up against which villains though, and if you don't think a hero could handle it themselves, then suggest pairs of heros (probably C, B or A class) working together.
Alright, with all these formalities out of the way, let's read some combat!
Crabrante smirked down at the brat who'd drawn on his chest with permanent marker, and lifted his claw to strike. "Die!" He shouted, though before he could actually bring his crab claw down to strike him, he was hit in the face by a bicycle that had gone flying through the air.
"Justice Crash!" The hit caused Crabrante to back away as the kid ran in the other direction. "Thank you!" He shouted to the man dressed in brown and black leather armor and a green helmet, "No problem kid, get going." He said with a smile as he walked over, picking his bike up and getting back on as Crabrante glared.
"Just who the hell do you think you are!?" He questioned. "Names Mumen Rider, local hero." He introduced himself. Crabrante laughed, "A hero huh? You think you can defeat me? I'll be washing your blood off my claws in moments!"
"We'll see." Mumen Rider said, kicking off and beginning to ride in circles around Crabrante. The crab man continued to glare, trying to snap Mumen Rider up, but unable to from the speed of his bike riding. It wasn't super human, but more than the average speed most people could reach, and far beyond what Crabrante could hit.
As Mumen rider circled him, he thought fast about what to do. 'No sense trying for a direct attack, that armored crab shell of his will block anything I could send at him,' Mumen Rider thought to himself, 'but his eyes and his lower body are exposed. Without the element of surprise he'd block Justice Crash easily, but maybe..'
"Quick running 'hero'! Fight me if you think you're so tough!" Crabrante shouted, thrashing wildly around him despite Mumen Rider circling outside of his reach. When the crab man had tired himself out with the frantic attack, Mumen Rider changed course, riding directly at him now that his movements would be too slow for him to block or dodge.
"Justice Combo!" He shouted, jumping from his bike. The bike sailed over the ground and crashed hard into Crabrante's unarmered crotch, making him double over in pain as Mumen Rider grabbed both of his long eye stalks, yanking them down to the ground with him.
Crabrante wailed as he was pulled to the ground by his eyes, and was in enough pain that Mumen Rider had enough time to get back up, get to his bike, and put some distance between him and the monster grab before he finally got back to his feet, which still shook with pain, "I'll tear you apart!" He shouted, his voice a few octaves higher.
Crabrante charged Mumen Rider, who rode at him at full speed until he was within striking distance of him. Mumen Rider threw his weight down, knocking him and his bike to the ground. This let him avoid the strike from his claw, and trip the crab as he and his bike crashed into his legs at full force.
Mumen Rider got back up, putting distance between them again. Once again, Crabrante got up, panting now. 'I'm wearing him down.' Mumen Rider thought, 'but how the hell do I finish a battle like this?' it was an important question, but Mumem Rider wasn't going to flee just because he didn't have the answer to it, and started charging the crab again when it began rushing him.
Thankfully, Mumen Rider wouldn't need to discover a means of lethal force to finish off Crabrante; as one found him instead. The sharp point of Stinger's spear cut through Crabrante's armored shell in one strike, letting him thrust through the monster easily.
Mumen Rider skidded to a halt before he would have crashed into the hero, class A rank 20. Mumem Rider parked his bike and saluted him, "Thank you for arriving when you did, not sure how long I could have held him off."
"According to the security camera that let me know there was a fight to show up to, you didn't just hold him, you did a pretty great job fighting this guy. Not bad for a C class. What rank are you?" Stinger asked. "C class, rank 100." He said, "I've only started quite recently."
"Rank 100?" He questioned, "Now that's a crime. I know C class heroes at rank 50 who couldn't have handled this thing. Come on, let's report this guy and see about getting you moved up a notch or fifty." He said, slapping Mumen Rider's back.
Mumen Rider smiled, nodding, "Yes, thank you sir." "No need to call me sir, it's Stinger." He said, lifting his spear over his shoulder after shaking the remaining crab man guts off of it.
"So, you say you want to work with me?" The crime boss said, arms crossed on the desk. The man across from him was odd looking, but not the strangest he'd ever seen, not by a long shot. Horse-Bone, C class hero currently undercover, nodded.
Despite being C class, he was highly trusted by the heroes association, and had been allowed to undergo this investigation with the help of another C class hero and a B class hero as well. Only Horse-Bone could speak directly to the crime boss though, as he was the only one who wouldn't be immediately recognized as a hero.
"Well, you'll of course have to prove what kind of muscle you've got, but judging from your appearance I can assume you're quite strong. You might just start off a bodyguard, but with loyalty and performance, you might have a desk like this of your own one day, and your own territory in town." the man said with a smile.
His smile melted away when he heard the commotion from outside the door. Someone was attacking his guards outside the door, and form the sound of it his guards were losing. He realized that the horse man before him didn't seem phased by this at all, and made the connection.
"You son of a-" Horse-Bone cut him off, slamming his hooves down on the desk to flip it towards him, then kicking it with both legs to pin the crime boss against the wall with it. Moments later the door opened and his assistance, Tank Top Tiger and Jet Nice Guy, entered the room.
"The area is secure," The B class cyborg said. Horse-Bone nodded, gesturing to Tank Top Tiger to help him move the desk and get the crime boss from behind it. Before they could however, a vibrant pink and purple light began to radiate from behind it as the man's frustrated grunts became an intense wail.
The desk was thrown off of him, and instead of the crime boss they'd seen before, now stood a tall anthro fish man wielding a flog and dressed in what appeared to be bondage gear. This was not what any of the three heroes had expected, but all kept their cool.
"Thanks for the help 'heroes'," He laughed, "A little push was all I needed to reach my transformation. Behold the true form of the Fish of Darkness!" He cackled, and moved fast, getting behind Tank Top Tiger and lashing at his back with such force that he was sent into the wall.
The Fish of Darkness turned to Horse-Bone, spitting some kind of egg at him. The horse man dodged to the side as Jet Nice Guy threw a punch. His strength hadn't been able to obtain more than rank B, but the cyborg could still strike with three times the power of the world's strongest boxer.
Unfortunately, Fish of Darkness could move faster, and avoided the punch, tripping him up and slamming him into the ground with his flail. Horse-Bone tackled the fish man, striking him with his hooves before kicking him across the room to Tank Top Tiger, who lashed out with a fast moving tiger punch.
Dazed, the Fish of Darkness couldn't avoid being grabbed and lifted by Tank Top Tiger, who threw him across the room to Jet Nice Guy, who finished the monster off with a Jet Uppercut. The punch sent him into the roof, leaving an impression of his body on ceiling before he fell back to the floor.
He was cuffed and placed with all his goons, guarded by the three heroes while they waited for a transport to come and pick them up. Another just well done by the lower class heroes doing their part.
Okay, let's address the glaring holes here. I'm aware that the heroes association exists only because of Saitama's actions against Crabrante, this story is founded on the assumption that it exists without him, and existed before him.
Yes, I absolutely made up the moves of Justice Combo, Tiger punch, and Rocket Uppercut. Certain heroes from this don't have listed movesets, so I took some liberties and made them up based on what little we have seen.
Also, I'm aware none of the S class heroes were in this chapter, but can you honestly any of S class needing to go up against Crabrante or Fish of Darkness?