Esdese from Akame ga Kiru! stands to be counted as one of the most powerful humans alive by her pure physical power alone, but she also holds in her body one of the most powerful weapons ever produced and is in complete control of it. She has been outright stated as being so powerful it will require an entire army and ten wielders of weapons similar to hers to even have a chance to beat her. She is also an insane sadist, who enjoys torturing and destroying her enemies and has ordered the populace of entire countries to be buried alive and massacred on a whim. The entirety of the most powerful of the protagonist team fighting her at once were unable to even scratch her and she was holding back during their fight.
Gilgamesh himself proves more than a match for an entire army, when he takes out Iskander's massive army of Heroic Spirits with a single swing of Ea.
Maka Albarn from Soul Eater kicks the shit out of a huge group of thugs who were attacking her partner, Soul, all by herself, in Italy on one instance. Being she's committed to only killing evil humans whose souls are no longer human and have become Kishin eggs, however, she merely leaves them unconscious.
Black Star is said to do this quite often too.
Pretty much all of the Hokuto and Nanto warriors of Fist of the North Star can quite easily mop the floor with entire legions of punks and soldiers.
Hiraga Saito from the The Familiar of Zero light novel/anime series stopped an army of 70,000 for about two days by targeting the commanders and flustering their movements. His predecessor destroyed an army of 1000.
Guts from Berserk is called the "Hundred Man Slayer" because he tore apart the good part of an entire company of Blue Whale Knights, which numbered at least a hundred men, singlehandedly. And this is before he took up his calling as the merciless demonslayer known as the Black Swordsman. Guts, at the time of this writing, has left this title far, far behind in his personal body count of both men and demons, cementing his status as an all-around Badass.
At one point during the Griffith rescue arc, an enemy commander insists to his nervous men (who are waiting to ambush Guts and the Hawks) that the one hundred men story must be an exaggeration, and no one could possibly do that. To give a proper impression of just how doomed the commander and his men are, Guts is currently running on pure apeshit Unstoppable Rage after seeing the state that Griffith was in following his year's worth of Cold-Blooded Torture, and is tearing apart every Midland guard between the Hawks and the way out. Yeah. Nice knowing you, fellas.
When the Knights of the Holy Iron Chains encounter the trails of bodies left behind by Guts on the entrance of Misty Valley, it takes them a while to realize it was all done by one man with one weapon, not a whole army with cannons and other heavy artillery.
Ogami the Lone Wolf once killed 70 other samurai in one fight. This in a manga that's supposed to be realistic...
In Shadow Skill, it is said that a Kuruda mercenary defending his friends and family is a match for a thousand soldiers. Gau Ban proves this in the assault on Blorahan by taking out the entire Soulfan army by himself.
In Dragon Ball, Son Goku defeated the Red Ribbon Army, the strongest army in the world at the time by himself. At age 12. When he told his friends, who had come to back him up the look in their faces was priceless.
Everyone in later Dragon Ball and all of Dragon Ball Z can be considered this.
Special mention should go to Dr. Gero's androids; he was the mastermind behind the aforementioned Red Ribbon army, and consequently decided to beat Goku at his own game in retribution: creating one individual with the power of an entire army. Androids 19 and 20 were formidable fighters even after the Z Fighters' Training from Hell in preparation for them, Androids 17 and 18 required the Saiyans to go through even more Training from Hell in order to change which end of the Curb-Stomp Battle they were on, and Android 16 was strong enough to hand 17 and 18 their heads...though he was never an adversary to them, or the Z Fighters for that matter.
And, of course, we have Dr. Gero's mightiest android, Cell, crafted with his evil and the Z Fighters' cells. His imperfect form is capable of fighting evenly with 16 and can grow stronger with every human he absorbs. His perfect form, meanwhile, is capable of Curb Stomping all of the Z Fighters at once ...except Super Saiyan 2 Gohan.
In the second season of Mobile Suit Gundam 00, Graham Aker/Mister Bushido has a "One-Man Army license", carte blanche from the Federation government to do whatever the hell he wants as part of the anti-resistance task force. Unlike this trope however, while he easily could go around wiping out whole bases, he mostly just ignores orders and has inconclusive duels with The Hero. Members of the group behind the Government Conspiracy, unsurprisingly, also have one.
Gundam 00 Awakening Of The Trailblazer introduces Captain Shaman and the Mobile Armor Gadelaza, which carries over a hundred and fifty GN fangs! The sheer speed at which he racks up kills is mind-boggling.
For that matter, any of the Gundam Meisters (or the Sol Braves squadron, made up of six of the best mobile suit pilots in the world), who are shown taking down entire enemy battlefleets single-handedly throughout the final fight (it says something about the sheer overwhelming numbers the enemy possessed that this hardly slowed them down).
Kira takes the cake really. While everyone else there (and Kira too at times) still had back up, for the majority of Seed and Destiny Kira was the only active pilot of his team with only their battleship as back up. He still came out on top, even when in Destiny he was fighting two other armies at once.
For those without Nico Nico accounts, the video is of every kill Amuro made in the original series. 142 on-screen kills. And since some of those were Mobile Armors or battleships, the actual number of Zeon soldiers he killed is quite a bit higher. This all took place in a couple of months, remember.
From the same series, Char Aznable. His claim to fame before appearing on screen? Soloing five battleships in a single battle. He's so good that he didn't take Amuro Amuro's skills (as opposed as his Super Prototype) seriously until he pulled something similar (namely taking down nine mobile suits and two warships, with three other MS and another cruiser dealing with the White Base, in less than three minutes).
Mobile Suit Gundam Wing's five Gundam Pilots have to be this by default, since each was sent down to Earth by himself with orders to kick the Alliance's butt, and none of them had any idea that the others existed until after everything started. So you can imagine the kind of carnage that ensues once they start working together...
When the Redshirt Army gets wiped out by Precia's Mecha-Mooks in the first season of Magical Girl Lyrical Nanoha, Chrono steps up to the plate... and proceeds to mow down Precia's welcoming committee with little effort much to Nanoha's astonishment, then tells Nanoha and her friends to handle the out of control Jewel Seeds while he blazes a trail to Precia on his own.
The other characters eventually gain this trait in later seasons too, soloing waves of Mooks on their lonesome. For example, Signum once demonstrated her new upgrade by showing up as the only reinforcement against an invading squadron of Gadget Drones, and destroying all fifty of them in one shot.
Earlier in the series, Hayate had wiped out four groups of 48 of the same Mooks with a single spell. With one hand figuratively tied behind her back.
Vagabond has "Musashi and the Seventy." The Yoshioka swordsmen are not all Faceless Goons (in fact at least three have full names and a number have internal dialogues), but EGADMiyamoto Musashi is right up there with Guts; it ends up catapulting him from a local celebrity to a national figure.
Nanatsu No Taizai Each individual Holy Knight is considered the equal of a thousand men. When the northern barbarians invade with an army of three thousand men, two knights working together are able to wipe the army out with minimal effort. The new generation of Holy Knights is even stronger if only because they use demon blood to boost their powers.
Minato Namikaze in Naruto is the target of a "flee on sight" order from the opposition during the war. Later, he takes out an entire force by himself. He was also the fastest ninja alive, repeatedly facing A and Killer B (the 2nd and third fastest ninja alive) multiple times and beating them, even in the first encounter A-at his maximum speed, couldn't touch him.
Note what that said. It was not a "kill on sight" order like (allegedly) some modern intelligence services have with regard to highly wanted terrorists. Flee on sight. Minato's mere presence was enough to send the enemy running. And now he's back.
Sasuke was considered this during his training with Orochimaru. He defeated 1000 Oto Ninja without killing any of them or suffering so much as a scratch.
In the Fourth Shinobi World War arc, Kabuto resurrects a bunch of these to fight the good guys, including many of the former Kages and the real Uchiha Madara.
Sandaime Raikage was able to single-handedly match a a force of 10,000 for several days, though they ultimately killed him. He could also go toe to toe with Gyuki and fight him till both were exhausted.
Naruto is one himself now, with each of his clones being a Kage level threat. And then there's Physical God Pain, who soloed the entire village of Konohagakure by himself.
Madara Uchiha annihilates most of the relatively undamaged Fourth Division by using his powers for a Colony Drop x2!
Even before that he was fighting them with his usual Sharingan arsenal-he was untouchable. Even one of his Katons was so strong that a whole army of Suiton users had to counter him.
Obito Uchiha defeats the entirety of the First Division using Gedo Mazo, getting the sealed Kinkaku and Ginkaku like he was taking a summer stroll.
Pretty much every upper-tier ninja qualifies as this, to be honest. The Third Hokage (a likely example in his prime) believed that Orochimaru could take on an entire country by himself; Sasori claimed to actually have taken on an entire country by himself; Deidara has multiple high-level explosives on his person and is a living nuclear-esque bomb (his suicide attack has a radius of ten miles); Pain- either as a collective or just a healthy Nagato- is capable of taking on an entire ninja village and destroying it, and is evenly matched to a six-tailed Kyuubi...the list goes on.
Each division is a minimum of 200 members. However, each Shinigami Captain is considerably more powerful in battle than every member of their respective squadron put together. However, even by the standards of captains, Aizen and Yamamoto stand out. Aizen is capable of defeating multiple captains, even when ganged up on and yet even he was nervous about taking on Yamamoto in one-on-one Shikai combat. In Bankai, Yamamoto is The Power of the Sun.
The Espada are so powerful only shinigami captains can fight them. The highest ranks of the Espada cannot be defeated by a single captain and required group effort from even captain-class characters to defeat. Starrk and Barragan were confirmed to be examples of Walking Wasteland even before they became Espada. However, like the captains, they also were not strong enough to handle Aizen's strength.
The combined might of the entire Royal Guard is stronger than the combined might of the entire Gotei 13. There are only 5 members of the Royal Guard. There are over 6,000 members of the Gotei 13.
Ichigo has a unique level of power for any character. When he hit his peak, he was curb-stomping Aizen. And the final arc confirms it was only a fraction of his true power.
The Stern Ritter are capable of fighting captains. As a result, sixteen of them were able to defeat just under three thousand shinigami (there are around six thousand in total) in a matter of minutes, including several captains, and they only took five casualties in return. Even the captains couldn't handle them.
Elfen Lied's Lucy, in both the anime and the manga, was known for gaining a massive headcount (literally), as in the escape scene at the beginning of the story.
Saiyuki's Hakkai killed one thousand demons with nothing but his own hands and a small knife. Unfortunately for him, this led to a Karmic Transformation because "bathing in the blood of a thousand demons" turns you into a demon. Unfortunately for everyone else, this makes him a lot more powerful. And he's the Team Mom!
Seiten Taisei (Goku's Superpowered Evil Side) and Nataku count as well. In the latter's case, Nataku's father considers it an insult that general Kenren's army could substitute for him.
In Saiyuki Gaiden, in their last stands Kenren takes out a good number of the weird genetically engineered animals that Li Touten created, and Tenpou the same for a lot of men (he arguably would have won if his glasses hadn't been knocked off).
Black Lagoon. Revy, Ginji, Fabiola, but especially Roberta. That chick wields a .50 caliber sniper rifle one-handed, and she could shoot you with her belt buckle.
Zoro took out a hundred bounty hunters in Whiskey Peak on his own. They surrounded him while he was standing on a rooftop, and he disappeared and proceeded to beat the crap out of them shortly after. This was very early in series, and he's gotten vastly stronger since then.
CP9 also counts; they've been trained since childhood to become superhuman assassins. But none fit more than Dragon-in-Chief Rob Lucci, the only antagonist who has ever been able to match Luffy's Charles Atlas Superpower…and even surpass it. The only thing that made him lose to Luffy was the latter's greater willpower, and their battle took every last ounce of it; when Luffy finally managed to knock Lucci unconscious, he could barely muster the strength to speak.
Fisher Tiger also counts, climbing the Red Line and raiding a Marine base by himself. Not to mention freeing all the Celestial Dragons' slaves. Note that the Red Line is over 6 miles tall. So Fisher Tiger climbed a mountain taller than Everest, then immediately took down an entire Marine base (including oneshotting a Vice Admiral) without taking a moment to rest.
At the time of Dracule "Hawk-Eye" Mihawk's introduction, Gin referenced him as being one man destroying 50 ships. This was not a difficult feat for Mihawk; he just did it because they were there, and he, the world's greatest swordsman, was bored.
Capone Bege ate a fruit that made him a fortress for minature armies, when an object or human goes out of the barrier, the object or human expands to normal size. So, he is literally an one man army.
The three Marine Admirals are the only human antagonists pre-Time Skip who could put the Straw Hats on the losing end of a Curb-Stomp Battle; all three of them are not only ferociously powerful, but have Logia Devil Fruit powers. Aokiji casually froze the ocean solid and then incapacitated the three strongest members of the crew at once, Kizaru casually knocked down a multi-story tree with one kick and then almost killed the Straw Hats before Silvers Rayleigh showed up to subdue him; and Akainu…oh, he takes the cake.
Admiral Akainu is the series' penultimate example during the war. Once he'd made up his mind to kill Luffy, not even the combined efforts of Jinbei, Ivankov, Inazuma, Crocodile, Marco, and pretty much every remaining Whitebeard pirate could do anything more than slow him down. And this is AFTER getting into a one-on-one fight with Whitebeard and living to tell about it. To top it off, not only is he still standing at the end of the war, but shortly thereafter he goes after the Blackbeard pirates...who promptly get the hell out of dodge when they hear that Akainu is in the area.
And Luffy's grandfather Garp asks Fleet Admiral Sengoku himself to hold him back when that happens, or he will kill Akainu, singlehandedly. Sengoku being the only person Garp trusts to be physically capable of holding him back.
The aforementioned "Dark King" Silvers Rayleigh was Gold Roger's second in command. He is seen easily handling an Admiral (Kizaru), and says the only reason he is not also taking down a Pacifista and protecting Luffy's crew simultaneously is because of his age and his lack of action in several years, not to mention he would not call more attention to himself by straight up killing an Admiral.
But Gold Roger himself, is probably the best example of this, as it was once stated by Garp, that he single-handedly defeated the entire army of an unknown country, all because they have insulted his crew. It was stated that only one that could equal him was Whitebeard, and that was back when the latter was in his prime and before the number of fights he was in took their toll on him.
Several characters in Hellsing (Anderson, Walter, The Captain, Seras), but most notably Alucard. Not only is he capable of single-handedly slaughtering hordes of ghouls or normal humans but when he has his full power unleashed, he is capable of summoning familiars made from the souls of every being whose blood he has drained, numbering in the millions.
He may have lost this ability while recovering from the quantum indigestion caused by eating Schrodinger. Not too big a loss considering that the only one he needed to use it on is dead.
Sousuke from Full Metal Panic!. Even at the age of eleven, he was shown to have single handedly killed at least fifteen trained soldiers during one battle in a war zone without breaking a sweat. Gauron saw the aftermath, and it was Love at First Sight.
It's also interesting that there are moments of allegory in which they literally are a one man army; the most prominent example would be China single-handedly defeating both Japan and Germany with just a wok.
And then there's Hungary taking out the entire Prussian army by herself. Probably another instance of allegory, but still pretty freaking awesome.
Any high level mage probably counts as this. The most extreme example is Jack Rakan, who, well, just look. Even Fate prefers to run rather than fight him.. Nagi also counts, being even more powerful than Rakan.
Rakan: Are you sure you wanna take us on? I mean, you don't have that much firepower. Commander: We have two entire fleets of airships and over three thousand elite soldiers beyond the next rise! Even you can't possibly... Rakan: Like I said: are you sure? You don't have that much firepower!
And now Negi is worthy of such title, slaughtering through hundreds if not thousand of shadow demons while barely slowing down. Other members even lampshade it by saying that he doesn't leave any enemies for them to fight.
Fallan Denzell from the criminally short-livedDouble Arts is rumored to have taken on and defeated 50 bandits single-handedly. Eventually, though, he sets the record straight: it was actually 500.
The Super Robot Genre can be summed up as "a single guy in a giant robot is more than enough for vanquishing an entire alien invasion / interdimensional Eldritch Abominations / whatever wants to destroy the world this week."
Mazinger Z is interesting because it both plays straight AND subverts the trope. The Humongous Mecha eponymous is certainly powerful enough to trash an entire army (and in the original manga, in one scene Kouji estimates its strength can be equal to at least five percent of the total combat strength of the world's major military powers)... but when Kouji has to fight more than two Robeasts at once, he struggles (Mazinger versus Devilman movie) or loses (Mazinger versus Great General of Darkness movie, Mazinkaiser...). Its sucessors, Great Mazinger and UFO Robo Grendizer have been seen taking on entire armies and winning (the former in one manga chapter penned by Gosaku Ota).
Even then, Ryoma Nagare stands out, because of that time when he didn't have his mech... and stormed the enemy capital by himself anyways. And would have won if the bad guy didn't summon four kaijus to help.
Any Evangelion Unit or Angel is this, due to being all but invulnerable to anything up to a antimatter positron cannon (powered by a nation's entire power supply). Or nukes. Asuka futher proves this point by, in End of Evangelion, lobbing a BATTLESHIP into an entire tank and artillery brigade. A description of the very first Angel might give a better image: "...chewed through a Tank Battalion in four minutes flat, destroyed three dozen aircraft and took more firepower than the entire Pacific Fleet could dish out, right up to the use of Strategic scale N2 weapons. The UN and JSSDF threw everything they had at it and barely slowed it down at the cost of over a thousand lives and half a billion US dollars". This was one of the weaker Angels.
In the 17th episode of Durarara!!, Shizuo Heiwajima takes on over a hundred supernaturally powerful and durable Knife Nuts at once using only Good Old Fisticuffs. He doesn't just win — he Curb Stomps them while suffering nothing worse than a number of shallow cuts and scratches.
Jacuzzi Splot most definitely qualifies, what with raiding eighteen Mafia speaksies single-handedly within one night. Given that each hideout usually has at least a dozen mooks or so...that's about....yeah...
In Rurouni Kenshin, Kenshin is perfectly capable of taking on numerous mooks and dispatching them with pure speed. He doesn't have the title "Strongest of the Patriots" for nothing. One of his former allies Tani, a corrupt Isshin Shishi states that an entire military unit wouldn't be enough to handle him.
Also, Sanosuke took out 200 mooks while simply relieving stress, and he's far outclassed by Kenshin.
Really, most of Kenshin's allies and enemies fall under his trope.
In Fullmetal Alchemist, King Bradley proves himself to be this during the battle with the Briggs soldiers; for one thing, the guy takes down a tank by himself.
With a sword, no less.
Greed is able to take out just as many, if not more, Central soldiers and he destroys a truck with his bare hands. Helps that he can make himself Nigh Invulnerable.
The State Alchemists are viewed as such in-universe. The Ishbal war had been dragging on for years until the State Alchemists were sent in. It ended, quickly, abruptly and messily.
And the viewers get to see just why when Roy takes on Envy in a Roaring Rampage of Revenge. For some perspective, Envy is an Ax-Crazy shapeshifter, possesses an insane Healing Factor to the point where it's practically immortality, and can transform into a ten-story-tall massive green monster. Roy is very, very, very pissed. The battle is completely, utterly one-sided and takes Roy all of some thirty minutes of constant Kill It with Fire, with a side of flash-broiled eyeballs, to reduce Envy into a pathetic worm-thing. Note that it took that long in large part because Roy wanted it to be slow. And turning into that giant monster form proved to be a literally huge mistake, as it just made Envy a bigger target.
Shou, the main character of Akumetsu manages to take down 50 people unarmed before being subdued at one point.
In Beast Wars Neo, Big Convoy was told to be the "One Robot Army". However, since most of the time he's seen leading a group of Maximals, it could be just an Informed Ability.
Inazuma Eleven, Fudou tries to be this during the third season's match with Korea's Red Dragon. The result? He pisses other member off. This happens again with Brazil's captain.
In Ah! My Goddess, Lind the Valkyrie's job description is essentially that of Heaven's own One Woman Army. Belldandy has the skills and potential but not the personality to be the same.
Satan Hild could potentially qualify, one of her smaller clones that only had a thousandth of her full strength could her its own when Lind attacked her, and she was mostly just defending herself till she convince Lind that World Gone Madwasn't her doing.
A Certain Magical Index actually has a few: namely the saints, the level 5 espers, Knight Leader, and Ollerus, just to name some (proven by Accelerator and Misaka, the #1 and #3 espers respectively, both taking out large military groups on their own.) Supplementary material explains that this is how you achieve the rank of level 5 Esper; it denotes an Esper powerful enough to function as a One-Man Army if not a Person of Mass Destruction.
InuYasha: Naraku has a tendency to send out armies of youkaimooks against his enemies. This is problematic for several reasons.
Inuyasha has an Infinity+1 Sword which is legendary for its ability to slay a hundred youkai with a single swing. That claim to fame turns out to be based on its weakest power.
Miroku is capable of being a One Man Army due to Naraku having cursed his lineage to be carry a hole in their right hand that acts like a black hole. It will eventually consume Miroku's life but, in the meantime, he's learned how to harness its power as a weapon.
Gintoki in Gintama can be this when he feels like it, which is pretty much never. It's stated to be one of the reasons no one bothers Otose in Kabukichou. Most people would rather deal with the nose-picking lazy bum living above her shop instead of the White Demon.
Other characters, including villains, are shown to be this as well, Okita, Hijikata, Katsura and Takasugi (usually in flashbacks), Housen, Kamui and Jirochou have all mowed down mooks by the truckload.
The anime short Kigeki centres around the Black Swordsman (not the same one from Berserk), who is hired by a young Irish girl to take out an army coming to attack her village. The Swordsman obliges, and calmly slaughters all 200 fully armored knights on horseback in a few hours. Then he eats them for good measure.
In Shaman King Hao (Zeke) becomes so powerful with Spirit of Fire that he destroys an entire naval fleet designed to kill him and a shaman with a spirit stronger than Spirit of Fire. Then he becomes the Shaman King. Instant annihilation if he even looks at you.
Amidamaru also fits. He killed an entire army that came to capture him. Of course, he dies right after.
Oga: Get this through your thick skulls! The one you are fighting right now IS YOUR KING!
The Four Beasts in Toriko. The Mounturtle alone took a barrage of the combined might of the Human World, which could raze 10 million square kilometers (enough firepower to destroy all of Japan 27 times over) without a single scratch, the Gaoh avoided orbital bombardment from satellites while destroying the 100 meter long Mammoth Tanks, the King Octopus Kong destroyed said tanks with ease, and Invaitdeath just melted everything with his poison. Then the Four Kings show up and defeat these beasts in the span of three chapters.
Riki in Ginga Nagareboshi Gin. He might have been the only dog remaining in Ohu for now, but he keeps fighting bears all by himself as if it's easy hunting.
Many mages in Fairy Tail could count as one; Mystogan has destroyed branches of and those who are supporting a guild that was attacking Fairy Tail at the time, however only one has done this feat "on-screen". Erza Scarlet once noted that her hardest fight was against 100 knights. In the Pandemonium game of the Grand Magic Games, she once again defeated 100 foes. Live before an entire stadium audience this time.
Levi from Attack on Titan is claimed to be as effective as an entire brigade. This is proven true during his battle with the Female Titan who he inflicts serious damage on, when highly trained soldiers died fighting against it and failed to even injure it.
In Break Blade there is Girge. When you look up One-Man Army in the dictionary, his picture pops up. This guy can single-handedly take out some of the most advanced Golems, fight an elite troop of five and kill every single one of them with a Golem that has only one arm and leg left and smash the highest ranking soldiers of the enemy nation twice without breaking a sweat. And that's only nine-to-five stuff. His being such a Badass is the only reason he is allowed on the battlefield because of certain...personalityissues of his.
In Magi - Labyrinth of Magic, a Dungeon holder is one on their own due to the power of the Djinn they wield, especially Sinbad and Kouen due to the fact both of them hold multiple Djinns. Muu is a notable example since he is a Fanalis and cannot use magic but makes up for it for through sheer speed and strength. Aladdin is most definitely one as a Magi. As proven during the war in Magnostadt, he could have wiped out the entire Reim army if he wanted to.