One-Man Army/Fan Works

Examples of One-Man Army in Fan Works include:

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.
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.
Eri from Eri's Game underwent of taking down Mooks by herself upon becoming from fashion designer to butt-kicking activist. She kept this status even in the Reapers' Game.
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.
Drunkard's Walk: Strongly implied of the individual members of the Warriors, the Superhero team of which main character Doug Sangnoir is a member in his home timeline: the team, which is no larger than a dozen members, is a strategic force which can be deployed against entire national militaries.