The Hulk personifies this trope; he can spend an entire comic battling one superhero or villain, but when faced with the entire army of them then he takes them out like flies. Conversely if Hulk is on a team, he never seems to pull out quite the same levels of power/rage.
Spider-Man also has this habit to a lesser extent. He has fought the Fantastic Four and X-Men more than once and holds his own rather well despite the fact that individual members can and have done well against the wall-crawler in one-on-one fights.
The Hand, a group of elite ninja in Marvel Comics, is almost nothing but cannon fodder. The willingness to die seems to be more important in membership consideration than skill, considering how many hundreds (perhaps thousands) of these guys characters like Wolverine and Elektra have waded through. These were, at least in part, the inspiration for the Foot Clan, below.
Their earliest appearances, during Frank Miller's run on Daredevil, more or less codified the trope. Daredevil and Elektra would periodically face whole armies of Hand mooks and win fairly easily, then the Hand threw a few elite swordsmen at them and gave them a serious fight, and a little after that they produced a named super-ninja for a downright difficult boss fight.
The trope was later somewhat handwaved in a Wolverine comic "...[the mooks] have to be careful they don't chop one of their own by mistake. While I can hit anyone I please."
Speaking of Wolverine, the Hand brutally averted this one once by ambushing him with enough ninja to cripple his movement and leave him defenseless, allowing the Gorgon to chop his head off. He gets better, then plays the trope straight for six issues of glorious payback that practically wiped the ninja clan out.
A Silver Age JSA/JLA crossover had the Thunderbolt fall under the control of Johnny Thunder's evil Earth-One counterpart. While the Thunderbolt is one of the most powerful beings in the universe, when fighting the JSA, it was easily defeated when it split itself into six beings to take on each member. Justified because its power was spread evenly between all six bodies.
One The Flash storyline had a Speed Force enhanced bunch of Ninja going up against various Flashes and other speedsters. They realized almost too late that the more ninja they took out of the action, the faster the others were getting...
In a Flash vs. Quicksilver video, Flash and Quick seem to be running and fighting at the same speed. Finally, Flash gets pissed and circumnavigates the globe in a few seconds to lay the final blow.
The Marvel Family's powers work like this; The more that are active, the more their powers are divided amongst them.
It goes back and forth for them. Sometimes they're splitting the same power source, sometimes they each have their own.
Played with in a recent Runaways comic where Kingpin faces the heroes with an army of ninjas (more Ninjas then usual, according to one kid). During the fight, Molly (a superstrong girl who was very upset about punching Punisher, who had no powers to protect him, and had sworn off fighting anyone without powers) asks if ninjas had powers so she could fight them. She is given the answer, that, because they were ninjas, they counted as double, the implication being that heroes in the Marvel universe cut loose when fighting ninjas.
There's a Ghost Rider storyline that justifies this. Basically Lucifer splits himself into 666 different bodies; when one body dies, the remaining ones gain more power, until only one remains with all of the Devil's hellish force.
In The Negation #11, Obregon Kaine reminisces on a lesson from his training days as he watches hundreds of superpowered Australians thoroughly fail to defeat General Murquade: "It doesn't matter if you're fighting ten enemies or a hundred...just worry about the one you're killing now!"
Some supervillains have discovered, to their misfortune, that this cuts both ways. Juggernaut vs an entire team of X-Men? A city-wrecking battle in which the individual X-Men are injured, trains are derailed and buildings fall down. Juggernaut and Black Tom vs Cyclops? Cyclops runs rings around them while his internal monologue digresses about military history. Total property damage: One exploding pickup truck.
The Wrecking Crew embody this trope since their leader the Wrecker splits the power of his magical crowbar among the Crew. The Wrecker by himself is usually a serious threat. He has given a (weakened) Thor the fight of his life and later held his own against the New Avengers. The Wrecking Crew, Depending on the Writer, are either serious threats or joke villains. They can go anywhere from being able to beat down Hercules to struggling with the Punisher to getting curb stomped by the Runaways. The Wrecker is actually aware of this trope, but willingly splits his power anyway since the Crew, occasional treachery from the team's Evil Genius aside, is like family to the Wrecker.
On the subject of Doomsday, this trope tends to work in his favor as well, especially with those who uses him right - he's trashed two iterations of the Justice League, an iteration of the Suicide Squad and, in his early days, mowed through an army of Green Lanterns. He's practically ground to a halt when Superman steps in.
Subverted in an early issue of Teenage Mutant Ninja Turtles. Leonardo does battle with practically the entire Foot Clan and gets his ass kicked. Although, he did manage to put up quite a fight. This is actually an example of the Law breaking down as noted above, as Leonardo was increasingly worn down by one wave of Foot ninjas after another, while each successive group of ninjas was fresh.
#57: 100 armed Yakuza soldiers hopped up on MGH against an unarmed, civilian clothed Matt Murdock. Even the FBI agents who have the situation under surveillance know they'd just be in his way.
Ninja Army vs. Bullseye.
In full effect on James Robinsons' World of New Krypton arc in Superman. One Kryptonian? One of the most powerful characters in the DCU. 80,000 Kryptonians? So much canon fodder.
Inversely, how do they show off their power? They all go off and beat Doomsday into the ground.
Repeatedly invoked by multiple superheroes when they face a large gang of Mooks. Spider-Man, Batman, Captain America and The Punisher have all been surrounded by assorted groups of street thugs, ninjas, terrorists, convicts, etc., and almost always come out on top. Another variation on this trope was used in an early Spider-Man comic where three police officers burst in to help Spider-Man against a large gang of thugs. The cops are almost as effective against the overwhelming number of hoods as Spider-Man himself.
The X-books X-over "Second Coming" was made of this trope and Badass Decay. One Nimrod class sentinel nearly wipes out the combined X-Men and Hellfire Club. An army of them is nearly cannon fodder. Not to mention a combined force of Bastion, Stephen Lang, Bolivar Trask, William Stryker, Graydon Creed and Cameron Hodge getting taken out.
G.I. Joe often uses this trope, especially when dealing with the feuds between various Ninja-clans associated with either the team or the Cobra. Good example from America's Elite #26, where Snake-Eyes and Scarlett battle several dozens of mook-ninja's with great success. When nasty bad guy Firefly tries to escape, Scarlett tells Snake-Eyes to "Go, I'll take care of these losers", even though there are still at least a dozen left. Of course, the battle between Snake-Eyes and Firefly is epic in every regard. During the original Marvel run, issue #91, when Larry Hama was still writing the script, there was a slightly more plausible version, where Snake-Eyes, Scarlett, Jinx and Timber face-off about twenty Red Ninja's. First sixteen go down easily, whereas the last four manage to cause grievous wounds to both Scarlett and Jinx and even cut up Snakes a bit, before going down.
In Rising Stars, this is literally true, in that whenever a special dies, his power is divided among all the surviving specials, making them stronger. As the body count racks up over the course of the series, it goes from where, at the start, a few non-powered mooks could easily gun down dozens of low-powered specials to the point where, near the end, any one special can take out entire armies.
In the fourth issue of the third Avengers series, the team had become massive due to all the reserves being called in to fight Morgan Le Fay three issues prior. The team, which by then consisted of over forty superheroes, was called out to face the low-level supervillain called Whirlwind. Whirlwind basically danced rings around the Avengers, who kept tripping over each other and accidentally hitting their teammates instead, and got away laughing. By contrast, when Whirlwind later faced Justice and Firestar by themselves, they were able to defeat him easily.
Played straight during Infinite Crisis with Superboy-Prime: he tears through the gathered teams of Doom Patrol, the Teen Titans and the Justice Society with ease, but is easily spirited away by the Flashes save for the Golden Age one. Later on, he battles an army of Green Lanterns, killing nearly 50 of them, only to be stopped and put down by the Golden Age and Modern Age Supermen. Conner Kent, who is nowhere near as strong as Superman can fight him a lot longer mano e mano than anyone can during a dogpile.
Despite each individual member having their own power source (which, technically speaking, all drain from the same source, which is however essentially infinite), the Green Lantern Corps is painfully prone to this trope.
In Secret Six vol 2 issue 6, Strix very easily takes care of Riddler's goons in 4 panels in issue 6. The Riddler just stands there completely dumbfounded.