In The Immortal Game, Twilight Sparkle, after gaining the power of all five of her friends, pretty much becomes a physical god. At one point during the story, she single-handedly fights the strongest mortal bladecaster alive, his entire personal regiment of guards, and the avatar of a god more powerful than every other cast member combined. She does this without breaking a sweat.
In Uninvited Guests, Hitsugaya, after having fit of insanity, charges into the entire forces of Hueco Mundo and manages to traumatizes everyone, although he got caught and imprisoned. Played for Laughs, like everything happens in that fic.
In Dragon Age: The Crown of Thorns, Alim Surana, the elven mage, takes the crown. Granted, he needs a boatload of fresh blood (like, say, the one from all the fresh dead in Ostagar, humans and darkspawn alike) or a huge load of lyrium to let loose (read: hold off the whole darkspawn horde by himself for a while), but he manages to take out most enemies in every other situation as well.
In Ace Combat The Equestrian War, a foreign pegasus ace, Mobius is a fighting force equal to an entire squadron and he's capable of fighting and often defeating multiple enemies alone.
Commandos on both sides in Tiberium Wars, due to a combination of training, technology, high explosives, and heavy armor (GDI) or cloaking (Nod). Unconventional tactical approaches also help; i.e. the Nod commando kills a Guardian APC crew and then sets the vehicle's remote gun turret to fire automatically to force some reinforcing troops into cover, where she kills them in seconds from a flanking position. The GDI commando, meanwhile, clears out an entire room of Nod soldiers by using an optical cable to locate them through an air vent connecting their room to the elevator shaft he's climbing down, calculating firing angles through said air vent, and then sniping the Nod troops from the elevator shaft with his rail carbine through the vent's thin metal walls.
In the Kung Fu Panda story Making the Cut, Jo fights off the entirety of Master Keen Lee's 52 kung fu students though with much effort and injury. The author later said that the scene was inspired by the Crazy 88 fight scene from the movie Kill Bill.
Col. Flix from Q Basic Gorillas can take out dozens of Ice Chimps all by himself all at once, whereas two or three Ice Chimps at once is an awful lot for the other Gorillas to handle.
In The Wizard In The Shadows, certainly in the later part of the story, Harry is one of these. At Helm's Deep he casually harness a thunderstorm, and deals out massive amounts of Shock and Awe. This is after he set up a deadly barrier that obliterated anything that came through it. Around the entire fortress. And holds it. For two hours.
Harry and Ginny with the Sword of Gryffindor. Harry vaporises a Mūmak. As a side effect. And that was the weakest attack. Ginny creates three tornadoes. Of fire. Destroying the physical forms of three Nazgūl and tens of thousands of orcs, wargs, trolls and easterlings.
Attelus Kaltos of Secret War was a skilled apprentice-assassin in his own right at the start. But has gotten even more Badass as the the story progressed. Able to fight on even ground with enemies with twice his experience and win despite being badly injured and exhausted (more due to his rampart cheating, but still) also able to fight and defeat many a Mook group singled handed.
In Rise Of The Red Eclipse, Centurion Chris-Z9571 is a ten foot tall walking mass of armor, muscle, and hidden weaponry with a stealth factor capable of taking on enemy shock troops with ease, jumping out of gunships and landing without a scratch (in which he usually creates a small crater where he lands), taking on small armies by himself (and occasionally with his own personal squad of marines), and taking on some of the hardest hitting villains of the series in hand-to-hand combat, usually either hurting them enough to retreat or beating them to the point that they have to be evacuated.