The Order of the Stick: Vaarsuvius gives one of these to the ancient black dragon after accepting the soul splice. Comics # 652 and # 653 pull a complete reversal: Vaarsuvius decides to take the fight directly to the Big Bad Xykon with the soul splice's "Ultimate Arcane Power". Several rounds later, Xykon manages to literally beat the power out of Vaarsuvius.
The fight between Roy and Xykon is a straighter example.
O-Chul vs. Redcloak. The paladin takes the ~17th level cleric's Disintegrate without blinking and proceeds to Smite Evil Redcloak's eye with a blunt metal bar that he ripped from his cage. Then O-Chul takes away Redcloak's holy symbol, temporarily negating the vast majority of his spellcasting ability and leaving the cleric no choice but to flee.
Belkar does a fairly good job of fighting off both Bozzok and Crystal while the gang fight the Thieves Guild, but once Bozzok leaves to chase Haley, Belkar takes down Crystal in what appears to be two rounds without even letting her get a hit in.
Haley vs. Crystal is one as well, as Haley deliberately waited until Crystal was getting out of the shower to attack her (so that Crystal would not be wearing her standard magic items), and then took her by surprise. Crystal is on the floor in seconds, and then Haley kills her.
Deviant Universe: Omega vs the Thunder Force during the Omega Rising finale. Omega won the battle, but lost the war.
In Fans!, there is the last battle between the Fans and the FIB. What should have been a curb-stomp battle the other way around is reversed when a literal Deus Ex Machina turns it into one of the most one-sided battles ever, resulting in this: http://www.faans.com/index.php?p=1290
If the stories are true, Liu Feng from How I Killed Your Master did nothing else. He described his experience as "I don't know what it's like to hit a man twice."
Homestuck has lots of these. Let's enumerate, shall we?
Dave still manages to maintain his Bad Ass status after verbally curbstomping Tavros (aka adiosToreador) in a reverse-trolling of epic proportions later in the series. (Technically in a flashback, but whatever.)
The showdown between Vriska, Gamzee and Eridan. The winner? None of them. Kanaya turns up in all her rainbow-drinker glory and wipes the floor with all three.
The long awaited battle between Vriska and Tavros. Vriska slaps Tavros in the face then runs him through with his own lance.
After defeating a whole bridge full of Imps, you'd think Vriska would be strong enough to fight Aradia, right? You'd be wrong.
Anyone versus BecNoir- Bro, Davesprite, John, Rose, Dave, a thousand Aradiabots- ends with them getting crushed. The only exceptions so far have been God-tier Vriska rolling straight 8s on her dice (which didn't count anyway because it was in a doomed timeline), John when he was running from PM and fighting in a dream bubble, and Prospitian Monarch, who has the exact same powers as Jack and is really pissed to boot.
Jake fights Dirk's training robot. The fight includes the robot slicing bullets in half in midair, performing a rocket-assisted suplex, and punching him so hard that his glasses fly off taking his eyes with them.
Dirk's rap-battle with Squarewave can hardly even be called a battle, as it is finished in only two attacks (by the same party) and the first didn't do anything. It's not even clear when Squarewave was supposed to get a chance to act.
Girl Genius has a Curb Stomp battle during the defense of Mechanicsburg. And immediately after as well, both performed by Gilgamesh Wulfenbach. We never knew he had it in him... oh wait, yes we did, just nobody else did.
In Sidekicks Darkslug dishes out heavy beatings to everyone he faces. In his first 'fight', he effortlessly defeats and kills three villains without even breaking stride. Even in the season finale, he still gives the Big Bad a brutal beating despite the latter owning one of the Triad Prana.
In season 2, Darkslug's sidekick-turned-superhero Lamia, as the hero Nightmare, dishes out beatings in the same manner as her former master, effortlessly taking out three villains in the span of three chapters. Special mentions go to the second villain on her menu, Metaroid, who she bifurcates with nothing more than a nonchalant backhanded swipe.
Nightmare provides another lesson to an army of Alex clones despite having drained her powers beforehand.
Quant destroys all the Regulars he faces in the Hide-and-Seek test (until he is challenged by Androssi) despite being restricted to only using 1 baang. Justified since he's a Ranker.
Jyu Viole Grace delivers swift deaths to almost all the Regulars he encounters in the Survival Round of the 20th Floor test.
During the Trustworthy Room game, Horyang crushes Rapdevil off-screen in his One-Winged Angel form, leaving him in a bloody whimpering mess afterwards.
Urek wallops Team Tangsooyook about when they meet him inside of a giant Shinheuh; to say he was barely trying would quite easily be an understatement. Justified since he's the highest active rankedRanker in the Tower and they're just Regulars. But not long after this, he's challenged by Rankers who he absolutely annihilates using not much more effort than he did to deal with Team Tangsooyook.
The Imperial Legion's invasion of Ziragalen in Second Empire. A more brutal Defensive Feint Trap has never been seen - complete annihilation of all Legion assault teams and near-eradication of retreating defensive teams. It helps their commander is an irredeemable General Failure and that they have been bred to eradicate independent thinking. EXTERMINATE!
U18 [read: Manga] Vegeta versus his Planet Vegeta native counterpart, who never left their home planet. Even turning into the Oozaru didn't help the poor guy.
U8 Cooler vs. U12 Future Trunks. Cooler takes his defeat hard. Part of this may have been Trunks saying that he wasn't worth killing.
The worst part is that, had he actually fought the Trunks that killed Frieza and Cold in Universe 18, Cooler would be more than powerful enough to beat him... but he had the misfortune of fighting the Trunks who had become so strong that he could effortlessly defeat Imperfect Cell and the androids.
U18 Goku vs U10 Massato and U13 Kakarot vs U6 Kat.
Bra vs Zangya.
Subverted with Gast, while many expected this to be a curbstomp battle, but the battle begins with Gast hitting the floor.
Doubly subverted when Gast faces Frieza, at first its more-or-less a curbstomp battle for Gast, because Gast stomps Frieza's first form, then loses to the second. When they fight again, Gast stomps every last one of Frieza's forms.
Universe 18 (main) Goku vs Android 17. One-Hit KO!
U17 Cell vs U6 Bojack. Bojack's only attack that even works is his Energy Bonds, and Cell easily escapes and kills him.
When Zen Buu decides to take control of everything, and start giving everyone trouble (plus Vegetto is nowhere to be found), Gast casually stands there, tells him to stop, and then uses his psychic powers to suck every little bit of Zen Buu into a ball, completely trapping him and leaving him virtually powerless.
Buu's rampage itself counts. With just a few gestures, pieces of him start devouring the whole stadium, effortlessly restraining everyone. Even Ultimate Gohan and Super Saiyan Bra of Universe 16 were helpless to resist. The only ones who do resist it are the inhabitants of Universe 18, due to everyone- Gohan, Goku, Uub, Vegeta, Trunks, Goten, Piccolo, and Pan- combining their power into a barrier. Even then, it was about to fail on them before Gast intervened.
Surprisingly, U8 King Cold vs U3 Bardock turns into this once King Cold uses 100% power.
From Terra, the "fight" between Agrippa Varus and Catella Myrha. Catella moves in fast and Agrippa almost contemptuously slaps the knife out of her hand and Neck Lifts her. Certainly explains why she fled rather than face him a couple chapters ago.
Hilarious example in Turn Signals on a Land Raider. Baneblade turns up to a 700 point battle, starts bragging, gets creamed by a dozen Lascannon shots, and then realises that the Baneblade is 634 points and hence he has no other units on the board. One turn win for the Space Marines.