The Bob and George adaptation of Mega Man 5 had Bob clone every Robot Master from the first four games dozens of times to overwhelm Mega Man and friends. However, they're so weak as to have several taken one from a single charge shot. It could be handwaved by Bob using Dr. Wily's Mook Maker to spawn them rapid-fire and sacrificing quality over quantity.
The trope is also Invoked by a villain later on who arranged for Dr. McNinja to be cloned hundreds of times, so instead of the Doc being one man fighting his army of mooks, said villain would be one man fighting an army of McNinjas, thereby making the latter almost powerless.
You wanna fight? There's only one ninja left, and that means I'm death incarnate!
The page image comes from a scene from Sam and Fuzzy, where Mr. Blank takes on the mook legions of Mr. Black. Blank and Black are both blankfaces, elite ninja assassins, while the mooks are... well, they're mooks. As Mister Blank puts it about two minutes later: "Looks like I might have to change my name to Mr. Red!"
Justified in Fans! when Team Alpha faces the golems of the Order of the Dragon. The larger the summoned army, the more the Order members have to spread their power among the individual units, resulting in a frightening number of mooks that all drop with one hit.
Sluggy Freelance: In "The Sci-fi Adventure", the spaceship Torg and Riff are on is attacked by a group of xenomorph-style aliens in space. The crewmen shoot them easily in zero gravity outside the ship (when there are a lot of them), but one of them manages to infect one of the humans, causing one more alien to burst out of him inside the ship and go on a rampage. She starts killing crew members effortlessly by herself, until only Riff and Torg are left, at which point they find some big guns and shoot her down.
Dr. Beetle tried to pull this as he "put all his eggs in one basket" with one huge clank Mr. Tock and didn't even call for reinforcements immediately. This didn't work for very long.
Gil goes out alone, likely in full knowledge of this trope, to face off against an army of invading clanks. Naturally, he destroys some and forces surrender from the rest. The most hilarious portion of this sequence is that the invading army's general realizes what's going on and attempts to have Gil shot before he can do anything.
He did have a really powerful weapon Magnificent Deathray. It is elsewhere established that most of the crazy inventors who try to face off against armies with their lone death ray tend to meet a predictable end.
Later two airships full of Wulfenbach Stealth Fighters were chasing the Vespiary Squad. They even managed to kill or wound most of this crack team, but ran into a few Jäger generals and a Smoke Knight who in turn massacred them and made it look insultingly easy.
The Heterodyne monster held by the Corbettite order, one it escapes. The entire order of badass monks who have been confiscating and hiding weapons for hundreds of years, the airship fleet of the current claimant to the Storm King throne, and the main characters together can barely do anything until Agatha figures out its weakpoint.
Lampshaded in Captain Snes in a guest comic showing various characters respective ideas of heaven and hell. In Edge's heaven, he is surrounded by 9 obviously lovestruck Rydias. Edge acknowledges the readers knowledge that this would probobly be too much for one person, before stating "I have 3 words for you. Inverse. Ninja. Law."
Invoked, lampshaded, discussed, and finally both justified and played straight by Vehemence in Grrl Power.
Invoked by Vehemence personally, who stays out of the fight until every other super villain is beaten.
Lampshaded on his behalf by Sydney, who realizes that as the last villain standing and one who's refused to fight before now, he's probably the toughest and will take a lot to beat, with her loudly calling "inverse ninja!" on it.
It's then discussed when Maxima tries to invoke reality on Sydney's assumption, with Sydney countering by pointing out "what's worse, one hundred storm troopers or one Darth Vader?"
Finally played straight and justified when it's revealed Vehemence draws his power from vehemic energy...also known as violence in general, and he can get it just by watching. So showing up to a superhero fight and hanging around until the very end just makes him even more powerful and harder to beat by the time the heroes are done with everyone else.