What can Muggles do?

[Mechalich]

Surprised that no one's brought up the trope yet- it's at least tangentially related.

In most narratives, the watchword for the common folk is 'strength in numbers'. Or, to put it another way, 'quantity has a quality all its own'. Generally speaking, there are gonna be a whole bloody lot more of the common folk than the super-types (whether 'super-human', super-natural' or whatever), and humans, no matter how ordinary, are terrifyingly good at figuring out how to kill things.

Even Saitama, for all his game-breaking power, isn't outside the realm of possibility for mundanes to put down; at least, not entirely. It's implied pretty strongly that he still needs to breathe (cf. when he gets blasted to the moon) and he still sleeps, whether he has any actual need to do so or not. Put some decent surveillance on him (as far as I can tell, his travel speeds are still observable), sneak in and remove his CO2 detectors, turn the gas on, and leave. Even if he's totally immune to poison (which is certainly possible, given that this is Saitama), not having any breatheable air in his apartment while he's asleep is gonna do him in.

Muggles also talk to one another- in a modern setting, the level of problem-solving available on any subject that you can get enough people interested is ludicrous. Almost nothing that you can get enough (bored) people looking at will remain any kind of a secret, at least not for very long.

So... what can muggles do? Piss them off enough, or get them curious enough, and there's almost nothing they can't kill, almost no secret they can't discover.

... all of this with the caveat that yeah, there are power levels beyond what mundanes can have any perceptible effect on.

Strength in numbers is certainly a thing, but there are limits to how far that goes. In the most basic sense, if we're all fighting with swords, only so many guys can surround a person at once, if your hypothetical superswordsman can't be harmed by that many and never gets tired (or regens fast enough) then no number of people makes a difference. There's a seen in Sword Art Online where this point is made explicitly by having the stupidly over-leveled protagonist Kirito stand there and let some goons hit him for a while but he fails to take any damage because his auto-recovery is higher than their combined DPS output.

Functionally, there's a point at which extreme personal power becomes political power. To quote Petey from Schlock Mercenary: "I am a foreign power." Any being powerful enough to take on say 100000 - 1000000+ of their contemporaries just isn't playing on the individuals only stage anymore, they're a political entity in an individually shaped package. Exactly at what point this happens varies from system to system and setting to setting of course, but it is a thing and settings where such individuals exist are different from settings where they don't. And this is still different from settings where literal invincible gods like Superman (Saitama's a bad example because One Punch Man is a gag comedy, Superman works much better) walk among the masses and could, if they so choose, totally rewrite society wholly according to their own wishes - something Superman has done on numerous occasions in various alternate universes.