There are no rules. However, I must state this. Please don't be offended if you're a fan of any of these characters, this is only for the people who think they can beat anybody(basically any toxic fanboy/fangirl out there).
1. Mega Man X: X really only needs his buster considering it's good enough to take out hordes of enemies. He could use any fire attack to burn the group(considering his fire is hotter than the Earth's core). He could just stop time and proceed to kill them from there. He can turn invisible and silently kill them with his Shadow Armor. And that's not even mentioning X's Hadoken, Shoryuken, or Giga Burst.
2. Ryu Hayabusa: Hayabusa might as well just use his sword because the techniques he has are deadly. Let's see what he can do here. Either he stops time and eats them with the Art of the True Inferno. He could just summon a black hole that sucks the killers in. He can just teleport behind all of them and proceed to assassinate all of them with one swoop. Oh, and even if they badly damage Hayabusa, he'll just regenerate considering he's survived getting dismembered and deformed.
3. Kirby: Kirby can summon a fire dragon that eats the killers. Kirby can also use the Robobot Armor and proceed to play a game of Whack-a-mole(but replace the moles with the killers). He can just eviscerate them with a laser. He could also just use any of the OP weapons such as the Starship, Star Rod, Rainbow Sword, etc.(Considering I'm allowing Kirby to use every weapon in his arsenal).
4. Soma Cruz: Soma can just petrify them and break them while they are turned to stone. He could also freeze them and destroy them or burn them with a mountain of fire. He could summon a familiar to assist him in battle. Soma could also manipulate them with matter manipulation given they are made of matter. He can destroy the concepts of the killers and then proceed to murder them using rings of water.
5. Lightning: Lightning can decrease the killers' durability and strength and burn them with Firaga. She could also blind them and speedblitz them. Or she could just destroy them with light considering some of them are associated with darkness.
6. Doom Slayer: The Slayer doesn't even need his firearms, as his bare hands can rip out these guys to shreds. If he uses the arsenal he has, there won't be anything left out of the killers that's for sure. One BFG shot and boom, gone from this world.
7. Chosen Undead: He can create a shockwave and burn all of the killers while they're in a paralyzed state. He could summon lightning and electrocute them harshly and then freeze with absolute zero ice. He could also increase his stats and overpower them using stronger weapons. Or he could distract them using Aural Decoy and stop time, allowing him to throw spears of lightning at them.
8. Ingrid: Ingrid could shut off their minds and blast them with light. She can just pass through their attacks and trap them with Sun Delta, thus allowing her to electrocute with lightning. She could also send them back to the end of time, or proceed to create a storm that kills them.
9. Dante: Dante just using Alastor and his guns Ebony and Ivory, is enough to take them out. Dante could negate all the damage that was from them and use a powerful counterattack. He could also shoot out meteors, lightning, or fire to kill them. He could use Pandora to shoot them and turn them into swiss cheese. Let's not even mention his devil triggers, of which could likely turn all of the killers into nothing.
10. Sol Badguy: Not a lot of things from the killers' side won't work, because Sol can just resurrect and he can see through the illusions they have. Sol literally turns them into ashes. Or he could just bring his allies(Ky, Dizzy, Sin, the Valentines, etc.) and they all combine their attacks into a stronger one that completely destroys the killers' whole body structures.
I'm trying to think what Jeff The Killer's best feat is famously. Aside from fighting the Slenderman in crossovers he typically doesn't go up against dangerous adversaries. I think Jane The Killer is his most popular rival and she's just a woman with a knife.
It'd be very interesting to see how far you can juice him up. That being said, I don't know how well Jeff would do against a well trained SWAT Team considering how he often kills teenagers and runs away before the cops get involved.