Druss the Legend from David Gemmell's Drenai Series. With no training he kills six well armed veterans with a wood axe. Later he attacks a camp of forty raiders single-handedly and wins (Though this could probably be attributed to the fact their camp was on fire and their horses stampeding at the time). Throughout his life he goes through many such badass actions: Fighting a boxing champion to a standstill at the age of 17, ending sieges through single-combat with enemy champions, fighting hundreds of battles and campaigns, literately been to hell and back twice. But the greatest moment of Badassery is his death. 60 years old, poisoned and heavily wounded he holds the gates of Dros Denloch for a while against a horde of enemies, taking over thirty with before he finally falls.
Remo Williams - aka The Destroyer (the overall title of the series of pulp thrillers in which he stars) - is a definite example. Trained in the mysterious martial art of Sinanju, he's capable of running across water, dodging bullets, and feats of effectively superhuman strength (achieved through concentration and energy-conduction of a Use The Force-like kind rather than by bulging muscles). Most of these were carried over in a low-key way into the one film so far based on the series. Ultimately, he's revealed to have become an actual avatar of Shiva.
Rand al'Thor is a bit of a loose cannon, but as the Seanchan discovered to their dismay, he is more than capable of blowing their front lines back across several hundred miles of previously secure territory in the space of an afternoon. Even if he had help. And blew a few of his comrades up too, but hey, that's insanity for you. By the antepenultimate and penultimate books in the series, he not only is able to unwittingly affect an entire city, for good or ill, just by being there, but capable of feats unaided that make the previous, aided, efforts against the Seanchan seem mundane.
In the very first book, The Eye of the World, he taps into a vast reservoir of magic (the titular Eye) and uses it to annihilate a Trolloc army of maybe a hundred thousand that had been about to sack the country of Shienar. In this case he's more of a conduit than an agent, but he does at least manage to point the power in the right direction.
In Crossroads of Twilight, he takes out another hundred-thousand or so Trollocs, this time under his own power. Moreover, he is caught almost totally off-guard and is holed up in a farmhouse that is practically indefensible and is more of a liability than a bulwark. He does have some help from the handful of Asha'man with him, however.
In Towers of Midnight, he goes against several hundred thousand Trollocs, with only two personal guards. The sheer intensity of the power he shows drives every Darkfriend within a few miles completely insane. It's a rare series where the Chosen One, while insanely messed up, is clearly the Chosen One for a very good goddamn reason: he is actually so powerful and impactful to armies and the Pattern itself that you can actually believe talk of destiny in facing an ancient unkillable evil Dark One.
Close enough to what the Aiel call Lan, the most bad ass swordsman in the world.
Also Demandred in A Memory of Light, during the Last Battle.
Honourable mention to the Kingdom of Lancre, whose standing army literally consists of only one (normal) man: Shawn Ogg. Except when he's lying down. Of course, given that behind him stand Nanny Ogg (except when she is lying down), this may be the most powerful army in the Discworld. On the other hand, Lancre has Granny Weatherwax. She might not single-handedly slaughter her way through an invading army, yet she is still famed as invincible, ruthless, and terrifyingly competent. As an usurper and his shrew of a wife, a wicked godmother, the Queen of the Elves and an entire family of vampires Vampyres (to name only a few) have discovered to their misfortune.
Hogfather features a throwaway reference to an action figure called "Captain Carrot: One Man Night Watch". It's appropriate. Captain Carrot isn't technically from Lancre, but Copperhead, which is close enough that his father sent a messenger to ask Magrat Garlick for help with spelling a word. He's perfectly capable of slaughtering an entire army if he has to, but he's so good at talking to people he never had to.
Sam Vimes has stopped entire wars dead in their tracks just by being himself at them, and he gives orders to Carrot. This frightens people, and it should.
In the Honor Harrington novels, the main character is this on a personal level, and her third hyper-capable command, the second HMS Fearless, are both this. The first Fearless was an antiquated light cruiser interrupted on her way to the breaker's yard to be turned into a test ship for an experimental weapon. Having failed at this, she is shuffled off to a quiet assignment to run out the clock (not that it happens that way, but that was the intention). The secondFearless is a state-of-the-art heavy cruiser, with double the firepower of a conventional heavy cruiser, and she finishes the story by taking out a battlecruiser three times her size.
In The Executioner action novels Mack Bolan, Vietnam veteran turned vigilante, decimates the American Mafia using everything from frontal attacks with rocket launchers, machine guns and sniper rifles, to using infiltration and wiles.
There are two states available of Achilles: Achilles in His Tent and routing the enemy. Interestingly, Homer played with this trope: almost all of the Olympians stuck their fingers into affairs of mortals at the time, so Troy would be screwed even without Achilles, as it had its share of divine enemies, and the Greeks got their asses handed to them while Achilles was wangsting because his mother specifically asked Zeus to provide a nice background for his later return.
He often gets overlooked, but arguably the biggest badass in The Iliad is Diomedes. Agamemnon calls him a coward, so, just to prove his mettle, he goes and kills so many Trojans that Ares himself is compelled to intervene. Diomedes proceeds to send Ares crying home to daddy. Keep in mind that Ares is the fucking God of War.
Also, Patroclus. When the Trojans were getting too close for comfort, Achilles still wouldn't fight. Patroclus put on Achilles' armor and charged into battle. He, by himself, roused the Greeks and pushed the Trojan army back. They thought he was Achilles. Too bad he met up with Hector, an even more badass one man army. Even that's selling him short. Hector wasn't able to take him down alone, Apollo had to sneak up behind Patroclus and knock his armor off, and Eurylochus stabbed him in the chest first. Patroclus himself tells Hector he came in third at best.
All of the above pale before Ajax. Diomedes defeated two gods in one day (Ares and Aphrodite), and Patroclus routed an army until he lost his armor, but both were defeated by Apollo. Ajax, however, was never beaten in the Illiad, even by the gods. In fact, when Zeus forbids the gods from helping the Greeks (but not from opposing them), all the Greek heroes are driven from the field, one by one, except Ajax, who is wounded by several gods, but never stops fighting. How many times can you put "the combined efforts of several gods, while he had none to help him, failed to stop this guy" on someone's resume? He racks up a mook body count roughly equal to Achilles, fought Hector to a stalemate, and when he actually does die in later (now lost) poems? It's by suicide. That's right, the only thing badass enough to defeat Ajax is... Ajax. Wow.
Really Diomedes could have ended the Illaid a lot sooner, the dude managed to kill every person he fought against. The only reason he ever fails is because of divine intervention or due to his honor (like that guy he traded armor with). Took three Olympian gods to stop him from killing Aeneas and potentially ending Rome right then and their. He was so powerful that when Diomedes decided to fight Hector, Zeus himself sent lighting bolts down in front of him, because he knew that he can kill Hector (heck before this happened, Diomedes hurled a spear at Hector which would have killed him if Apollo hadn't magicked his helmet at the last second, but even then Diomedes was able to KO him).
Greek Mythology was filled with examples of this. Even Oedipus got in on it, when he unknowingly killed the king of Corinth (also his father) and his entire bodyguard, leaving only one survivor. The survivor ended up lying that they had been attacked by a gang of thieves, because no-one would have believed him if he told the truth. Jocasta said at one point that if you want to play Oedipus, play on his fears. The messenger that tells Oedipus he was actually adopted by Polybus is there to bring Oedipus back to Thebes, where he'll be rewarded for bringing the king (ie. ulterior motive). And the servant who confirms the tale is a slave, and at the time the testimony of a slave was only considered valid if delivered under torture. Layers upon layers of confuddled possible half-truths.
World War Z mentions in one of Todd Wainio's recounts a soldier who "was a monster with a two grand body count". They are zombies though—they aren't remotely strong against anyone with a gun, the range to use it with and decent aim, and they tend to come in crowds, meaning that high body counts are a foregone conclusion. The relatively light defense at Hope ended the battle when they created a zombie pile so big, the zombies couldn't climb it, without a single casualty. Though apparently quite a few of that monster's body count was done by hand; Todd recounted one instance where he picked up one zombie and used it as a club against a whole mob of other zombies.
He's a pilot, not a commando, but Wedge Antilles is an astonishinglygood pilot who, more importantly, survives absolutely everything thrown at him ever. The series mentions kill silhouettes painted on snubfighters—as well as having half of a Death Star, Wedge has so many TIES that they're rendered in red, so that one TIE silhouette represents twelve kills, because he's taken out too many to fit on the fuselage otherwise. In a later book, a character painting up a fighter is hip-deep in infiltrating the enemy as a bombastic Space Pirate, and adds a ridiculous number of kill silhouettes.
Then again, Wedge is the only pilot in the entire Star Wars Expanded Universe to have flown against two Death Stars and lived to tell the tales.
Armies have been trying to take Carnival down for three thousand years. It has never worked because she's one of these; she even takes out most of an undead horde before they finally get the better of her by force of numbers. And even that can't keep her down for long. It helps that she's a demigod, though.
Zhao Yun is recorded as single-handedly taking on Cao Cao's army in order to rescue Liu Bei's heir in the historical novel Romance of the Three Kingdoms. May or may not have actually happened.
In Changes, we get nine people (well, okay, seven people, a fairy, and a dog) versus the entire Red Court. Murphy does a very decent impression of Michael in the process.
Any of the Wardens also qualify. Dresden himself is generally no slouch in the combat department, but even he is struck by the difference between his usual battle magic and the skill, ferocity, and efficiency with which dedicated Wardens can go to town. In Dead Beat, the first time we really see Wardens at work, five of them scythe their way through dozens of zombies within minutes—and with Dresden zombies, think less Dawn of the Dead (1978) and more Terminator. It's mentioned that Morgan once got within twenty yards of The Red King, which entailed cutting his way through several Physical Gods in addition to what must have been dozens of regular vampires in the way.
And even the Wardens pale in comparison to The Senior Council. Summed up, this is a group of the seven most powerful wizards on the planet. Not seven of the most powerful. The seven most powerful. This group includes: "Injun Joe" Listens-To-Wind, a Native American shaman who is so proficient at spellcasting and shapeshifting he can curbstomp the native equivalent of an angel and make it look easy, Ebenezer "Blackstaff" McCoy (considered the magical equivalent of the Heavyweight boxing champion) who is so strong he can pull a satellite out of orbit and drop it on his enemy's mansion and casually and literally rip the lives out of hundreds of people at once. Taking the cake though is The Merlin (the title of the leader of the White Council, named after the obvious), who is generally considered to be the single most skilled and powerful wizard on the planet. He stops the army of the Red Court with one ward. This entailed keeping over a dozen Physical Gods held back long enough for his own army to escape, and he did it with one ward. As Harry (and Morgan) notes: "You don't become Merlin of the White Council by collecting bottlecaps."
Túrin could also count as this. He defeated upwards of 80 foes singlehandedly and killed the second most powerful dragon in the verse alone.
Fëanor was no slouch either, remember that one time Gandalf fought a Balrog and it was a tie? Fëanor fought all of them. Every Balrog. At once, and it STILL took them hours, and a being that was on par with Sauron, meaning second only to Morgoth himself, to take him down. And this is after chasing a routed army virtually alone for hours. There is a reason he was called the greatest elf.
A Bolo is effectively a One Tank Army. Earlier models get referred to as Continental Siege Units, while later ones take it Up to Eleven, becoming Planetary Siege Units, able to engage everything from a lowly foot soldier up to space battleships.
Allanon in Terry Brooks' Shannara series. In The Elfstones of Shannara, The Alliance against the Demons consists of the Kershalt Trolls, the Westland Elves, the Dwarves, the Border Legion Free Corps, and Allanon. That's right. The man is actually badass enough to qualify as an army whenever the leadership gets together, and is entrusted to do things like hold entire ridgelines by himself. Being the most powerful Druid ever probably helps. And then of course there are things like the Skull Bearers, and The Reaper which might also qualify for this.
Shardbearers are actually stated outright on several occasions. Shardplate makes them nearly impossible to harm, as well as enhancing strength and speed, and a Shardblade is a weapon that can cut through normal armour effortlessly and instantly kills if it passes through the spinal cord at any point (and it literally passes THROUGH living tissue, without cutting it).
Also Kaladin proved that, even without Shards, a Surgebinder can be a one man army. Even if said Surgebinder has almost no knowledge of how to use his abilities.
The books like deconstructing the trope. Sure, Shardplate makes a man nearly impossible to harm—but not actually impossible, and the entire enemy army will overwhelm them in time, meaning they require support from regular troops.
The dark nature of such absurd killing power is discussed more than once. Shardplate, Shardblades, and Surgebinding were all originally designed to allow humanity to fight Voidbringers. Now that the Voidbringers are gone, humanity has turned these weapons on each other, and it's rather horrifying how easily they slaughter mortal men.
Inheritance Cycle: Roran. It was one hundred and ninety three! Any of the Dragon Riders and elves probably count, but it's never stated explicitly.
In the Matador Series, Emile Khadaji is a literal one-man army, though he uses deceptive tactics to make it appear otherwise. He parlyzes 2,388 soldiers over the course of six months, then when he's about to be found out, he does the same to their commander, then turns himself in. As intended, it inspires others to start a revolution. "If one man can do this alone, think what all of you can do together."
Any mistborn would probably count, but especially Vin. At one point, Elend even promises a minor character "two armies". Vin, by herself, is the first of said promised armies.
Steel Inquisitors (created to kill mistborn and possessing many of the same powers, plus an insane Healing Factor) also certainly qualify, and a Koloss may not be worth an entire army, but a fully-grown one is certainly worth a company of human soldiers. Taken Up to Eleven with the Lord Ruler- it's explicitly stated that he could kill the entire population of his capital city (currently in a state of revolt) by himself given enough time but fortunately Vin manages to figure out his Achilles' Heel before he can actually start doing that.
In Harry Potter, both Lord Voldemort and Professor Dumbledore. To put it in perspective, Voldemort is seen to effortlessly murder a room full of Death Eaters in book seven, and still he's afraid of Albus Dumbledore. Who is, at that point, dead. But Voldy still gets rattled by the name when Harry mentions Dumbledore during their final confrontation.
Lionblaze from Warrior Cats, due to his Nigh-Invulnerability. One is example of this is at the ending of Outcast when he takes on a large group of cats who had been giving the Tribe of Rushing Water trouble throughout the book and comes out covered in blood- none of which is his.
In the Codex Alera there are numerous characters who, for whatever reason, are of such badassitude that they just handle entire armies on their own.
When the gates to Garrison are breached by the Marat horde, more than ten thousand strong and accompanied by wolves and herdbanes that fight beside their warriors, Pirellus of the Black Blade orders Amara to have the remaining legionares regroup on the top of the walls to prevent more Marat from scaling the fortificaions. When Amara asks him who will hold the gate, he responds that he will, and proceeds to do so. Singlehandedly. Against the entire horde.While getting impatient at the way the Marat seem to be holding back.
Giraldi explicitly calls Araris a "one-man Legion" in Cursor's Fury after he massacres dozens of Kalarus' Immortals singehandedly.
In Paradise Lost, the Son defeats Satan's entire army of rebelling angels on his own. For all of Satan's big talk, God and the Son are of course infinitely more powerful than him.
All You Need Is Kill: Played straight, deconstructed, and discussed. Both Rita and Keiji are clearly One Man Armies, and it's stated that without Rita humanity would have lost the war years ago, but they're not so powerful as to be able to save all their teammates, a failing which rips them up inside. Some of Keiji's army friends think that Rita is just a propaganda creation because nobody could possibly be that good. After Keiji breaks out of his loop, his incredible battlefield prowess causes his old teammates to regard him warily.
Loki Stormbringer (real name: Brian Gragg), chief enforcer of the Darknet in the Daemon duology. Each book ends with Loki versus a seemingly very much superior military force, the internal security of a clandestine military base in the first and an entire army of mercenaries in the second. Each fight turns out to be a Curb-Stomp Battle...in Loki's favor.
Hector: Right...Hmm. Dozer has a country named after him, but why is Gohvis so famous? Garovel: Regicide. Genocide. And punching a hole into a volcano that then erupted and wiped out an entire Vanguardian stronghold.
Violet from Valhalla finds herself alone against overwhelming forces three times in the story- The opening with the gangsters, the hazing scene with the recruits, and the climax. In all three moments, out of the lack of creativity to do anything else, she takes on the opposition herself and cuts through them like a tornado.
While A Song of Ice and Fire has a strong base of Reality Ensues to it that keeps this kind of thing fairly limited, Ser Gregor Clegane is a definite example. An almost 8-foot tall giant of a man with immense strength, he is able to wear armour-plate that most men would be unable to even move in, while still being fast for someone of his size. This is in addition to chain and leather armor underneath said leaden plate as extra cushioning. He is also able to wield a two-handed greatsword as if it were a dagger, allowing him the use of a shield in the other hand. The result is a man who can charge and break pike lines all on his own: now, just for giggles, add a very big horse. Worse, he's got the psychotic bloodlust to do just this whenever, wherever.
Ser Barristan Selmy is another low-key version. Just ask the troop of hapless guards who were ordered to stop him leaving King's Landing. He was no longer wearing his high-quality Kingsguard bling and was armed only with a dagger and staff. He won; they got schooled: simple as... And, he can't even be bothered to make a fuss about it, as the really memorable part of the day happened before that. Heck, Assassin's Creeding things is his thing; from rescuing the being-held-for-ransom King Aerys II from the heavily-defended Duskendale to later finding Aerys' own daughter, Daenarys Targaryen, in a besieged city on another continent... getting in and out of belligerent, armed camps that hold numerical superiority most would consider impossible to crack with minimal, but well-targetted, casualties is what this Badass Grandpa's good at.
The general rule of thumb for when facing even a single Valyrian dragonlord astride their dragon is... don't. Frontal attacks are suicide, however many hundred of people you have and however fantastic you think your spellcasting in the face of flames strong enough to make forrest fires blush will be; guerrilla your socks off, restrict their battle arena and/or bring a bigger dragon or two with you, instead.
Richard Rahl from Sword of Truth, shortly after acquiring the eponymous blade, is able to defeat thirty master swordsmen who have trained from dawn to dusk since childhood.
The Primarchs of Horus Heresy were made to be this and do they ever deliver. Their kill count can climb into the thousands and they can perform absurd feats of strength, from holding a foot of a Humongous Mecha up with their bare hands to swinging heavy battle tanks out of their way with hammers. To give a more direct example, even when attacked by surprise in cramped, close quarters by twenty armoured, heavily armed and prepared soldiers, alone, unarmed and unarmoured himself, Roboute Guilliman wipes the floor with the Alphas in less than a minute.
During A Mage's Power, Fairtheora guarded Ataidar's border with Latrot (half of the southern border) by himself. Kasile's father thought that anything more would be superfluous.
Such is Dengel's spiritual and magical power that he can swat any number of soldiers like flies with a single gesture.
James Bond, who has this trope as part of his job-he's sent into enemy territory to take down entire private armies or criminal organisations on his own. He's able to take on multiple opponents with little problems in hand-to-hand combat or gunfights, and always manages to demolish whole criminal empires by the end of the book. However, From A View To A Kill deconstructs this aspect-being used to handling huge threats alone, Bond goes up against several heavily armed Russian operatives by himself without any backup and barely escapes death.
In Babylon 5: The Passing of the Techno-Mages trilogy, Galen becomes the most powerful Technomage by the end of the third book and is able to wipe out an entire city block by himself, as well a the planetary defense ships. When confronting Kosh, the latter isn't certain he would come out on top if Galen chose to fight. Later, Galen goes to Z'ha'dum and not only survived, but also helps Sheridan survive (more or less) and shuts down the planet's observation network. He also manages to take on two rogue Technomages, who have sided with the Shadows and learned some dangerous tricks of their own. Compared to that, his feats in Crusade seem like child's play.
The Witchlands has Aeduen, whose insane Healing Factor makes him practically unkillable. Couple this with his extensive combat training, his lack of morals and his ability to manipulate other people's blood, and it's no surprise that in a Mêlée à Trois between him and two armies, he's the one who comes out on top.
In Hurog, Tisala escapes after being tortured and is not exactly in perfect health when she kills a group of bandits, only armed with a tiny knife. Not that Ward is any less badass, he just doesn't get to prove it.