The Buffyverse-meets-Xmen premise of Heroes And Villains puts our heroine in a version of Sunnydale High where everyone has superpowers. Even so, Buffy's blend of strength, speed, agility and enhanced fighting ability allows her to achieve several very one-sided victories over her fellow students.
One of the biggest faults of crossing over two series with combatant protagonists is if the author turns every fight into a curb-stomp battle for the fandom they like better, even when it makes no sense, for example, for Edward Elric to be taking down 50+ Death Eaters at once.
In chapter 32 of The Wizard in the Shadows, Ginny flips out spectacularly at the Battle of the Black Gate. Channelling the power of what is hinted to be at least one of the Valar (and it being sustained by Gandalf and someone else), she obliterates the physical form of three Nazgűl and at least half of Mordor's army with three flaming tornadoes. Admittedly, Harry had just been killed, so she had a fair amount of Unstoppable Rage going.
In The Odyssey, due to an insane spree of luck during his battle with the former Immaculate Monk Beman, Ulyssian manages to leave not only victorious, but uninjured. Beman, on the other hand, left with a gash running across his chest and missing an arm.
Played straight once again later in the story, but with Ulyssian on the receiving end. After going to great lengths to sabotage and weaken his opponent Jael, he still looses quite handily to him, to the point that he would have died in the first few seconds had Jael not shown restraint.
In White Devil Of The Moon, Nanoha is able to kill Jadeite with a single Divine Buster attack. When Tuxedo Kamen and the Inner Senshi attack Fate and Arf on two occasions to prevent her from keeping them away from Nanoha, she is able to defeat them easily, although she suspects that things would have gone differently if they had been at their full potential.
An omake had Vivio defeat the Nanohaesque version of Chibiusa in ten seconds
Beryl vs. Miyuki. Despite what you'd might think, given that Beryl's a powerful being and Miyuki is a human, Miyuki does the curb stomping. She is a Takamachi after all. Beryl wins by sheer luck
Beryl vs. Hayate. Hayate learns the hard why why you don't bring a cannon to a gun fight
Lunar Heart powered Nanoha vs. Metallia. Nanoha wins by firing a Moonlight Breaker at Metallia from inside.
Earlier, we have Dalek Sec being utterly destroyed by KriemhildGretchen Thankfully, Sec gets better.
My Little Avengers: The first fight between the Avengers and the Dark Avengers ended completely in the latter group's favor. This is because Loki specifically chose them not just because their powers match and exceed those of the Avengers, but knew the Avengers well enough to hit their Berserk Buttons to throw them off balance. The Avengers Opponent Switch during the Final Battle in order to counter this and end up returning the favor.
In Dungeon Keeper Ami by Pusakuronu, Mercury invades the Avatar Islands - a blasted wasteland of dead rock and populated by thousands, if not Millions of undead and ghosts. Her initial attack is mildly successful. But Zarekos' counter attack has her on the ropes. At that same time Mercury discovers that Arachne, a rival Keeper with a grudge has deployed a force of several thousand giant spiders, and Keeper Morrigan who lands troops on Ami's beachhead, denying her access. What does she do? She pits the mindless undead against the spider army, allowing both to kill the other off. She then follows this up with a brutal Blitzkreig that utterly smashes Morrigan in the fiercest, most one-sided fight in the series.
What makes this such a Crowning Moment of Awesome is that three Keepers (Arachne, Alphel, and Nero) all stage a simultaneous attack on her dungeon almost directly after this, after weakening her forces and forcing them to fight on a battelfield of their choice. It's close, but she still prevails.
From Thousand Shinji, we have 35 Special Forces agents under SEELE versus four Rubric Marines. No contest. Justified as WH 40k tech is just that much more advanced than modern tech. Quite a number of the fights fall this way too in The Open Door, largely thanks to New!Chaos's Bigger Stick.
Seeing how Space Marines are One Man Armies with the Rubric Marines even more powerful since they're Animated Armor with force fields and fire bullets made out of hell which can tear normal Power Armour like tissue paper, it would take 600 Special Forces at least and even that with very heavy looses, so was this really a surprise?
Pretty much every battle in Eva Possibilities And Fusion Pandemonium is one of these. Bardiel, who absorbed Saiyan DNA through brief physical contact with Goten, is the first of the Angels to be the giver of a curb-stomping, effortlessly beating down even Gotenks and SSJ3 Goku... and then Goku & Vegeta fuse, and Gogeta effortlessly whomps Bardiel.
In Buffy's Friday the 13th, Jason hands one to Angel, stabbing through his shoulder and arm, and ending with him impaling Angel on the wall with his machete.
In Search Of Freedom, have the Straw Hat Pirates, ending up in Tenrou Island at the beginning of the battle between the Grimoire Hearts and Fairy Tail guilds, and getting them mixed up in the conflict. Since, this is the post-timeskip crew we are talking about, any fight where a crew-member is involved becomes completely one-sided to his/her favor.
This can be seen better, during the fight with Hades. In the original series, it took the combined efforts of team Natsu and Laxus to bring down the Dark Guild's master, and even then, the balance was still on Purehito's favor. Indeed, has it not been for the Exceeds destroying the Devil's Heart (the source of power of Hades), Natsu and company, would not have survived. Luffy, however, completely and single-handedly owns the dark mage, without even considering him as a challenge.
Luffy: Oi, old man, you're weak.
Christian Humber Reloaded has this when the main character fights virtually anyone, to the point at which the exceptions (such as him losing for most of the fight with Chridion) are less common than the straight examples, although some of this is due to a lack of description apart from the killing blows. In what appears to be the final battle, he manages to defeat Chaos Blade in three moves, then destroys the hordes of Chaos and much of the US army before killing the corrupted President without any indications of being injured or having any difficulty.
In the Twilight-Heroes crossover Golden-Eyed Pixie, Elle inflicts one on Jamie. It turns out that a vampire is not a match for electrical attacks. And then's there the fact that she gets doused in oil and blown up.
A Shadow Of The Titans: The Jade vs Kitty (Jade wins) and Raven vs Georgia (Raven wins) fights in the Tournament of Villainesses.
Gadjo versus anyone unlucky enough to piss him off always ends with a victory for him.
Rainbow is on the receiving end of one delivered by General Zod/ Prince Blueblood later on, before she is crippled by a Kryptonite necklace and tossed into a ravine.
In Horseshoes and Hand Grenades, Norio Eguchi, granted the power of Kamen Rider Den-O: Wing Form, tries to go up against a resurrected Gentaro Kisaragi. Unfortunately for Norio, he gets knocked down easily and nearly had his head cleaved off by a chainsaw had it not been for Sieg getting him out in time. Later on, Shun, given the power of Kamen Rider Den-O: Rod Form, goes off against Ryusei Sakuta and is also defeated... and then killed off.
Sick of Malfoy's bullying, Harry Potter-Tano challenges Draco Malfoy and three Slytherins of Malfoy's choice to a duel. To make things fair (and possibly to piss Malfoy off), Harry prohibits himself from using magic. He defeats the Slytherins simply by reflecting their spells back at them using his Lightsaber, then finishing them off with Lightsaber strikes.
Also, when Saber and Ben's Servant Avenger try to use their Noble Phantasms against each others, Gilgamesh shows up and interrupts the fight by unleashing Ea on both of them. He makes them bit the dust without even having to reveal himself.
In the Tamers Forever Series The sovereigns end up on the receiving end of a swift beatdown at the hands of ChaosGallantmon.
And during the final chapter of Silent Sorrow, Daemon gets the everloving crap beaten out of him by Gallantmon Crimson Mode.
In Dragon Age: The Crown of Thorns, the Sloth Demon's strongest form, like a werewolf, gets its rear handed to it, thoroughly, by dwarven commonner warden Faren Brosca, in single combat, even though it had been thrashing everyone else up to that point, including the protagonist. This is because the DC had made contact with it directly the moment it morphed and gained an understanding of how his Fade realm worked, then instinctively began to ignore all imposed limitation much like Sloth was doing. Flash Step battle ensues.
In the same tale, Uldred is killed instantly because the main cast doesn't wait for him to transform into the Pride Demon form.
Early in the story, there were several people who went up against Cid and Ensei, that were defeated in just one hit.
At the climax of the St. Galleria arc, after Ensei was able to give Judgemaster Cid proof of Vaticus' crimes, Judgemaster Cid sent his personal guard to assist The Heretics defeat the Archbishop Guard. The back-up easily defeated those that went up against them. Many who saw them were in awe.
When Veis and Villi were introduced, Veis defeated the leader of a group of would-be thieves easily.
The Firefly fic Forward has this happening at the end of the "Mosaic" story arc, when River, Jayne, and Mal take on and obliterate Ott's crew. Complete with Team Fortress 2 references.
Later on in "Second Interlude," Echo beats Jubal Early unconscious in the span of a single second.
In "Hunt," there's a cringe-worthy one where Kaylee fights a Hand of Blue. Kaylee is in a power loader. Needless to say, it ends fast and messy. For the Hand, that is.
Girls und Panzer
In Boys Do Tankary this is defied when the protagonists go up against St. Gloriana (which, in canon, ended with Miho just barely losing, after taking out all but one of her opponent's tanks). The male main characters could have won almost effortlessly, but hold back to see what Miho and the others can do, and ensure that they don't rely on the boys too much. Miho sees through the boys' claim of having mechanical problems and is less than happy with them.
Jackie Chan Adventures
Queen of All Oni: The fight against the sages in their shrine is entirely in their favor; Brothers Dark and Gray hold the advantage against the Shadow Hand and the J-Team the whole time, while Brother Light doesn't even seem to exert himself while continually tossing Jade and Hak Foo out of the inner shrine.
When Ikazuki temporarily usurped control of the Shadow Hand from Jade, it was greatly aided by his samurai wiping the floor with her ninja.
The Hero vs Mother fight lasts all of thirty seconds and ends in Hero's favor (which is quite impressive, given that Hero's hands were manacled at the time).
Uncle and Jade's first Wizard Duel outside the Section 13 Vault. She has the advantage for about two seconds before he overpowers her.
Hivefled: Gamzee spent two months imprisoned, tortured, starved, raped repeatedly, drugged into incoherency, and bled almost to the point of death, and still managed to deliver a curbstomping to both Equius and Eridan at the same time. Later on, Lereal attempts to lead the 250 Child Soldiers of the sufferist cult into battle with the empire, with predictably disastorous results.
In Perfection Is Overrated, Shizuru vs. Hitomi ends fairly quickly, with the new arrival launching a sneak attack on and destroying her opponent's Child. It doesn't help that the defeated combatant had posed a threat to Natsuki's well-being, and was largely unused to opponents who could actually fight back on equal terms.
As Shizune has the ability to reduce the effectiveness of people's powers and completely depower a single person, she's at a considerable advantage against the Himes, as well as her fellow SUEs in "A Common Enemy Without A Common Cause". On the other hand, her powers provide no protection against mundane weapons, enabling Natsuki to kill her with a handgun in the main story, and two police officers to do the same in "A Common Enemy Without A Common Cause".
Sleepwalker ends up fighting a Hulk being driven nearly insane with rage tearing through New York City. Nothing Sleepwalker does even fazes the Hulk, whether it's impaling him with a large barbed spear or electrocuting the Hulk by connecting him to the New York power grid. The Hulk rapidly heals from all of Sleepwalker's attacks, and while he doesn't manage to hit Sleepwalker too often, the blows he does land are quite literally the most brutal attacks he's ever felt. It's only because Sleepwalker noticed that the Hulk's anger was being increased by his Demonic Possession and tried to use his warp vision to free the Hulk from it that he managed to survive.
In Uplifted After the death of Galas'Yoad in the first book, Dieter Hertzer and crew of Sigrid use the goggles he'd modified for night vision to devastating effect against an Allied convoy moving through the desert at night.
Also during the Crisis, Lady Cripps the Pink led the forces of the Hyracotherium Republic to victory over the Celestine Junta. The Junta's forces were devastated, while the Republic lost exactly one soldier... who died from choking on a sandwich.
When the Founders were all in the cave during the Winter War, Platinum and Puddinghead tried to kill Hurricane, only for him to knock them both out with a single buck each.
Nihilus vs Luna, round one. Though it should be noted that the latter was exhausted and drained at the time.
God-mode Twilight vs anyone who isn't Titan himself (and that includes his Puppet avatars).
The second Battle of Ponyville is very clearly in Terra's favor, even when all the Mane Six and Luna are working together to try and bring her down. Then they reignite the neutralized Elements of Harmony and even the playing ground.
By chapter 42 Blackjack has gone berserk twice: The first time in Miramare Air Station where she only awakens after the carnage she wreaked unarmed was over. The second time her brutality made an allied Highlandervisibly afraid and her remaining enemies attempted to flee after her first two kills. They didn't make it.
Later, she fights Legate Vitiosis, who manages to completely No Sell everything Team Blackjack can throw at him up to and including cutting his head off with a starmetal sword. The only reason the story doesn't end right then and there is because he decides Blackjack isn't a Worthy Opponent.
In Jericho, there's one particular fight scene so quick that it's literally blink-and-you'll-miss-it. The main character, Jericho, is reading a billboard when a stallion comes up from behind him, they exchange words, and the guy catches Jericho completely off-guard with a punch. But the actual fight?
I gave the guy a side-neck chop as I pivoted and let him fall past me.
Some battles in Ace Combat Equestria Chronicles may qualify. Since they usually involve the Mirage squadron, their enemies have a hard time winning against them.
However, in chapter 11 of Ace Combat: The Equestrian War, Rainbow Dash is brutally curbstomped by Gilda. While being immobilized.
Pretty much every fight Rarity's in. She barely even breaks a sweat.
Subverted with Ran Biao, who actually proves a match for Rarity, though she still loses.
She seems to have picked this up from Lao Wu — the first match we see him in lasts all of a minute.
The Stars Will Aid Their Escape: Herald manages to defeat the Princesses quite easily when they're weakened by the eclipse. Shortly after, he's attacked by all the Element Bearers (and Spike) at once, and swats them all away with one hoof.
Of course, then Twilight recovers from the insanity he inflicted on her and together, the Bearers return the favor when banishing him back where he came from.
In the Pony POV Series, the Dragon-Hooviet War ended in one of these when Queen Tiamat entered the fray. Even by dragon standards she's huge, and after laying waste to the southern half of the Hooviet Empire, (willingly) stopped just short of completely destroying their capital.
Shortly before that, we have Proto-Nightmare Twilight vs Angry Pie. The battle is extremely one sided and by the end, Twilight, after being brought back to her senses, is left wondering how many times over she would've killed Angry Pie if she weren't immortal and Angry is left near catatonic.
In Melody's 7 Dreams/Nightmares chapter, Rancor, while visiting the past, gets out of hand and Mortis, who's watching her at the time, has to step in. He easily disables her by paralyzing her with acupuncture in a couple seconds.
In the second half, Melody's daughter, Sweet Music, steps up to defend a unicorn named Spell Circle from three colts who are bullying him. Despite being a blank flank filly, she beats the crap out of all three and sends them running home with a warning that if she catches them doing it again, she'll tell everypony in town they got beat up by a girl.
The war games exercise in Diaries Of A Madman ends up with the Night Guard winning against the Day Guard in a near-flawless victory after Navarone teaches them guerrilla warfare, to devastating effect.
In Nareto: The Scret of Shiobi, Emeraldia frequently gets into these. Interestingly enough, when she defeats Gaara with a single water attack, the narration tries to pass it off as a long fight.
In the Naruto fanfic Fan Fic/Team8, the Naruto vs. Sasuke fight in the Chunin Exams results in this due to Sasuke pushing Naruto's Berserk Button. Oops.
Virtually every battle from Naruto Veangance Revelaitons involving Ronan is this. Especially notable is the beginning of Chapter 4, in which Ronan severely injures Orochimaru in the first sentence and forces him to retreat in the second, despite only just having started the fight at the end of the previous chapter.
In Naruto Mysterious Power, there are a few minor fights like this, but the battle between Naruto, his harem and Future Sakura definitely count; Future Sakura pretty much overpowers them quite brutally. No, this troper is not kidding
Neon Genesis Evangelion
The fight between Unit-01 and Ramiel in the ninth chapter of ARSENAL qualifies as this.
In fact, every major battle that Shinji has fought with the exception of Chapters Eight and Ten qualify.
Kyon, Yuki, and Tsuruya in Kyon: Big Damn Hero pull this off against two dozen Yakuza, with the first two doing practically all the fighting.
Imperfect Metamorphosis pits a limitedly-powered-up-Marisa (was also supplied with twelve of Yukari's absolutely insanely over the top Spellcards), Mima, Yuyuko, Youmu, Utsuho, Satori, Orin, Flandre, Remilia, Sakuya and Yukari against Yuuka and Elly, who Yuuka brought in to even things up a bit after Yukari cheated and brought in 11 people to a supposedly one-on-one duel. Yuuka laughingly curbstomps the Scarlet Devil and Underground forces using one quater of her power (possibly permakilling Satori and Flandre, or trapping them in a Fate Worse Than Death), and then brings three quaters of her power to bear against Yukari and gets a draw where both sides are too damaged to continue, with Yukari in worse shape. The only battle she 'lost' was against Marisa, Mima, Yuyuko and Youmu, and even then she was only usin 1/4 of her power, and she had 'TeamHalloween' down to the ropes during the later part of the fight. Yuuka curbstomped most of the upper tier of the Touhou-verse at once without ever bringing her full power to bear.
Yu Gi Oh GX
In Dueling Unlimited we have a few duels that are completely one sided. The first is when Santa Claus shows up to take on a villainous mercenary. The next four take place when Anti-Heroes/Anti-Villains Bat, Boa, Raven, and Dove pull off their Big Damn Heroes moment and curbstomp enemy forces. All these duels double as Crowning Moments Of Funny.
In Our Mrs Monkey Luffy and Nami have gotten married. So when Absalom kidnaps Nami during Thriller Bark? Curbstomp ensues.
Nami: Why is there a hole in the... everything?
Luffy: Well, first I hit him in gear second, but then he turned invisible and I couldn't see him. But I knew he was in the room, so I used gear third to hit it.
Nami: Hit what?
Luffy: The room.
Tiberium Wars has this happen pretty much anytime infantry go up against a Commando. At one point there's a drawn-out fight between a GDI soldier and a Nod Commando that only exists because the Commando was quite clearly toying with the soldier.
Later on, this happens when the Nod army runs headlong into the Mammoth Tanks of the GDI 2nd Heavy Armored. Specifically, the one thousand Mammoth Tanks of the GDI 2nd Heavy Armored.
Fated.htm Fated has a different version of the climactic battle at the end of "Innocence". The Scoobies ambush Spike and the Judge at the factory. Buffy's blowing away the Judge with her BLOODY ROCKET LAUNCHER is followed by the Scoobies barraging the vampires with crossbows. Spike and the wounded Drusilla beat a hasty retreat through the sewers, leaving Spike's minions to get massacred.
Sins of a Colonial Empire has one: 30 Colonial ships from BSG 2003, including 6 Battlestars Vs. 1 GDI ship. the GDI ship, which incidentally is mentioned as one of the most powerful ships in the galaxy wins, suffering only 17% damage to their shields, even after being hit with 6 nukes!
In New Zork, the “Hitler Youth” ambush Paul and Ringo in the “alley” next to the Dakota Building. About ten seconds later, one guy has been slammed into the wall across the street, one guy has a garbage can crimped around him, one guy is upside-down in a garbage can, and one guy is hanging upside-down on a fence. Shortly thereafter, the fifth guy, after a brief moment of glory, recognizes just how outmatched he is and goes to help his friends.
The final battle in the warehouse.
When Ringo beats the huge warrior Mung in three seconds.
"I think I win. Can I please go to sleep now?"
Ringo's Crowning Moment of Awesome with the Brothers of Death (and it was kind of a double Curb-Stomp Battle, since he also humiliated Jeft by effortlessly neutralizing some of the toughest baddies he'd ever developed).
Most of the fights in Kaiju War Chronicles are fairly even, but some of them end up being this. Most of them involve Bagan during the "Bagan Trilogy" (Bagan vs Monster X, Bagan vs. Everyone, and Bagan vs Everyone: Fallout). First he utterly destroys Monster X with ease and the title of the other two isn't lying, it literally takes basically the entire roster just to stand a ghost of a chance against him.
In the Megacrossover Massacre: Fall of the Nosferatu, The fights between Diva fromm Blood+ and Damon (Vampire Diaries) Vampire Hunter D vs Alucard (Castlevania) and Hellsing's Alucard vs Simon Bellmont (Catlevania) are this trope.
The second battle of Markab in The Dilgar War sees the Dilgar First Strike Fleet, Second Strike Fleet and First Line Fleet receiving one by the Earth Expeditionary Force. Justified by the extreme toughness and short range firepower of Earthforce warships when compared to the faster but less tough Dilgar ones, the Dilgar being used to fight opponents with faster ships and less skill than Earthforce, Earthforce having brought an insane number of nuclear weapons and admiral Hamato having an intelligence operative telling him what orders Jha'dur gave to her forces, allowing him to place his dreadnoughts for maximum effect and launch all of his nukes at the one moment when the Dilgar were concentrated enough to be annihilable in one shot and with their point defense weapons away from where the missiles were coming from.
In XSGCOM, most of the Tau'ri - Goa'uld battles end up like this after the X-COM soldiers join the Stargate program.
It's perhaps easier to list the fights that The Prayer Warriors have that do not fall into this, but the one that stands out more than any is the battle against Hades. Hades is said to be Satan's "most elite servant", is able to revive all the other "false" Greek gods, and makes a Badass Boast about being stronger than God and Jesus and being undefeatable. The Prayer Warriors fall to their knees in prayer and he is immediately defeated.
This is a requirement for the duel against Guiche in any The Familiar of Zero fanfic, but it's up to the author to decide which side is the receiving end.