In the "Destroy the Arkbird" chapter of The Ultimate Ace, three Su-37s, specifically Yellow Squadron birds arrive to fight with Wardog and Mobius Squadron. Blaze charges forward to take them on singlehandedly while getting the other pilots to focus on their primary target... Which he somehow does, but exactly how he does it is not clearly described. Lampshade Hung in-story when the other pilots also are confused about how he pulled it off and Blaze simply says "classified" when asked.
Many of the fights in Christian Humber Reloaded are only described as far as Vash performing the killing move on the enemies, notably when Vash fight his corrupted self again in "Darkness Never Dies No Matter How Many Times You Kill It."
An Eye Of The Fox scene which depicts Kira calmly walking out of the Autobot's cave and proceeded to take out an entire forest it is still cool to imagine.
In The Last Spartan, when Tali is explaining the quarian Pilgrimage to Master Chief, she mentions a time when Kal'Reegar, himself on a pilgrimage at the time, returned to the fleet to give the Rayya's Captain a Brute Gravity Hammer as a return gift. Tali described Reegar as being victim of multiple suit breaches and other injuries, but when Chief asks her exactly how he got it, Tali only says "It wasn't easy."
In The Institute Saga, Thanos manages to assemble the Infinity Gauntlet with all six Infinity Stones despite all the good guys trying to stop him, then as he finishes assimilating the power it grants, he is faced with Spider-Man and Squirrel Girl. Cut to Superman finding Spider-Man muttering about how he was there, but still doesn't know how Thanos was taken down.
In The Bridge, a swarm of Destroyah babies overruns Ki Seong's restaurant and start eating and destroying everything, so she prepares for a fight. The next time we see her, she's somehow tamed them all and put them to work cooking and cleaning up the restaurant.
Chapter 43 shows Sif preparing to face down a giant sea serpent. When she next appears, she claims she took down five of them, and looks unscathed.
Peter Wisdom aka Regulus Black has apparently destroyed several of Voldemort's horcruxes.
When Doctor Strange first appears in Ghosts of the Past, it's apparently right after fending off an attempted invasion by Dormammu. As in, his clothes are still smoking in places.
Just prior to the fight in the Red Room base in Chapter 10 of Ghosts, Wanda and Dresden were involved in fighting off a bunch of especially tentacular demons in Japan, after a bunch of Otaku used the wrong book of magic in the wrong place at the wrong time. The results were, apparently, very messy, as were Dresden's unspeakable jokes about sushi and Japanese pornography. It also, apparently, resulted in a lot of gunk in Wanda's hair, much to her displeasure.
And shortly after said fight in the Red Room base, Thor and Steve march into the Kremlin, terrify Volodya (the Expy of Putin), and demand Harry back, Lukin and/or Sinister's heads on a platter, and the Red Room shut down, not necessarily in that order.
During the climax of the Forever Red arc, there are several fights — Avengers vs Winter Guard, Magneto vs Winter Guard, and Dark Phoenix Harry vs the rest of the Red Room — that are so one-sided that the story doesn't even bother to show them.
Occurs in Peter Chimaera's Digimon 2:Return of Digimon, where Digimon fights Evil Digimon. The first time Digimon one shoot kills him with his robot suit, only for it be reveal that "In Chapter 2, the evil Digimon is not really dead — IT WAS A TRICK!" Then, the second time, after "punching through the ceiling and fighting IN SPACE," the story in its entirety goes, "They fought for a long time until they were tired from the fight, and Digimon killed him with a lethal blow. "Impossible … I can not be stopped!" Then an explosion."
In Boys Do Tankary, the outcome of three tankery matches, one of which is against the apparent main antagonists, is mentioned over the course of six short sentences.Surprisingly, the heroes actually lose the last one.
A quite odd, untitled fanfic written in Polish (sporked here) does it spectacularly. In one scene, the good guys are sitting in a library, trying to find out how to defeat Voldemort. In the next sentence, the narrator suddenly states "Voldemort defeated!", hastily explains that he's been beaten in some unknown way and will definitely never rise again, then proceeds to the "Where Are They Now?" Epilogue.
The New Adventures of Invader Zim: In Episode 11, Tak is seen being attacked by a swarm of Zim's plant monsters. Later, she appears almost entirely unharmed and manages to track down the Central Root in order to aid in destroying it. The author merely says that it's because "she's just that good".
When all the people competing for the position of Jade's foreman are ordered to fight for the spot, Blankman knocks out all the others in under two minutes. Without breaking a sweat. Unfortunately, we don't get to see it — we see Jade telling them to fight, and then the narrative cuts ahead to afterwards.
The fight over Kuro's mask at the beginning of chapter 12. We only see the beginning of the fight, the rest is cut for the sake of the chapter's length.
The duel between Tarakudo and Hiruzen. One flashback ends with them about to fight, and another opens with Tarakudo just having killed him. And it seems he only won because Ikazuki defied his orders and helped him.
While the heroes are dealing with AntonMortimer, Tohru's sutras screw up Jade's tracking spell, and the Shadow Hand spend most of the chapter stuck on some kind of farm filled with magic animals and fighting them and their farmer. We only see bits and pieces of this, but it's implied to be one of the toughest fight sequences in the story.
Zelda's Honor has a rather prominent example at the end of Act 2. The battle between Ganondorf/Twinrova and the Talamir/Timner Eldritch Abomination builds up to be quite the epic showdown between three massively overpowered characters. Unfortunately, we cut away to the chase scene between Link and Barrachas; only to cut back later after the battle is over and Ganondorf's boot crushing the last remnants of the abomination.
Inner Demons has Rarity's fight with Trixie. The narration cuts away just before they go at each other; later scenes imply that Trixie needed Queen!Twilight's help to win. This is impressive given that Rarity is just a fashionista, compared to Trixie, who's trained in combat magic. The Element of Generosity changing into a suit of mystic armor probably helped.
In the Pony POV Series, Dark World has Minty Pie running after the heroes to help fight Discord, only to fall down into a series of caverns inhabited by some winter spirits that say she's their Chosen One in their battle against sun spirits. When we come back to this later, she's had a series of adventures and is facing down the evil Queen who was controlling both sides and manipulating her. Whether or not this works is up to personal taste
Hinata's performance in the unseen in canon Chunin Exam wherein she was promoted. Though she only gives Naruto a quick run down, she is happy enough with how she did that she boasts a little.
Kohaku Zoishin fought Minato Namikaze in the semi-finals of the Chunin exam that saw Minato promoted. The battle lasted about ninety minutes and left everyone who saw it serious impressed with the skills of both Genin, even if most of them don't remember Kohaku's name or appearance.
There are a number of small ones in Vapors because Naruto is the Hero of Another Story and having his adventures even when he isn't with Aiko. But there's one that must have been awesome when, while Aiko leads the benin against Gaara, Jiraiya teams up with the Third Hokage to permanently kill Orochimaru.
A Crown of Stars: After the conquest of Buenos Aires, there were two Time Skips to push the history forward, skipping to the arrival of the enemy fleet. An unfortunate effect of this was most of the campaign of liberation of South America (and thus a good humber of battles featuring Shinji and Asuka piloting a two-seater Humongous Mecha, Touji and Kensuke -Kensuke!- riding their own giant robots, Misato wearing a Power Armor and leading from the frontlines...) happened off-screen.
RE-TAKE: Fight scenes mostly occur off-screen. This includes Shinji, having knowledge of the future, disables Unit-03 without killing the pilot, and is stated to have broken all of its limbs. In the story's climax, Shinji's fight with the Mass Production Evas, most notably after Ghost Asuka finally decides to help him and they take the Lance of Longious. We get an Oh, Crap! reaction from SELEE saying they're finished and the Unit-01 is going to kill their Evas and we get seen afterwards indicating that Shinji and Asuka took them down easily, but we don't get to see the fight.
The final (extremely short) battle in the warehouse at the end of With Strings Attached is all sound effects and aftermath. That's partially because of how one-sided the fight was—the real battle was John and Ringo's attempt to get into the warehouse—and partially because the author hates writing fight scenes.
A written-out version of this fight, about a page long, does exist.
The Keys Stand Alone: The Soft World has some, again partially because the author still hates writing fight scenes:
John and Ringo's offscreen demolition of four ninjas, especially when they hang them on the side of the hotel with the words “Don't Fuck With 9000s” imprinted on them (also a Crowning Moment of Funny).
Ringo retrieving the countercurse from Gothmarik Citadel.
The first story in Peter Chimaera book of historical faFfiction is entitled Hitler: Extreme Nazi. Too bad the only thing extreme about it is its short length; it barely takes up one page, which is short even for Chimaera.