Not Quite As Planned: In a World where fighting with swords is the norm, Ichigo tends to use his feet, elbows and knees in fights. He even uses a Cero pointblank in a blade lock. Due to this he ends up with a defeated captain count of 6.
The epic-length Buffy fanfic Return To Normal/Beyond Normal, Buffy comes to the realization that Jack O' Neil is this. He might lose in 'fair' matches and tournaments, but in a real fight he'll START at kick to the groin and then he'll get nasty. Buffy puts this to use while fighting Kennedy during the final battle, where she bites off her nose before literally ripping her throat out.
In Spy Game Xander fights this way and encourages others to. After he questions Kennedy's teaching she decides to have him fight a Slayer in an effort to humiliate him. Before Kennedy shouts begin, Xander whips out a collapsible baton and hits the Slayer on the head, followed by a kick to the knee, a powerful blow to the head knocking her to floor, and ending it with a kick to the stomach.
Xander: Done yet?
Kennedy:(stunned) You. You cheated.
Xander: You think this a game? Treat this like a game and you'll get your ass beat. Just like she did. You want to win? You want to survive? Then you hit 'em hard. But, smart. In the right place. Right time. You box the ears. You spit in their eyes. You kick 'em in the balls. You don't let up. Ever. And when you think they're done, you fucking cut their heads off. Just to be sure. Power will only get you so far." You go in with that as your only weapon, and you'll just get everyone killed. You. Your sisters. Skill beats strength. Every time.
Chapter 17 has Night Raven, who calls Axe to grab Fluttershy's wings so he can beat her up.
This applies somewhat to the heroes as well. Punches and kicks are often aimed at a foe's neck or head, and it's not uncommon for them to finish off Mooks who are already falling from the sky or retreating.
The Battle Fantasia Project gives us Akiko Yamaguchi, aka Magical Girl Star Reverie, whose Mentor Mascot was killed in the line of fire before she could receive one of several power-ups needed to fight her latest batch of villains. By the time the next batch of villains has come around, Akiko is skipping her Finishing Move and using gasoline drums and Car Fu alongside numerous other tricks instead of more traditional magical girl tactics.
In the Italian remake, Sailor Venus is an advocate of this philosophy. Some of her best hits, dating back to the time of the anime, are rushing Galaxia while she was distracted to try and pour scalding hot acid in her lungs (Galaxia was distracted because Sailor Moon had just purified her, but Venus hadn't noticed it yet) and fill the van of Eudial's car with a lot of gelignite to detonate when she returned at the base (that's why Eudial's car exploded: as soon as it touched the water Venus set off the bomb). She later taught the same philosophy to the rest of the group and gave Naru the Fire Buster II (captured to Eudial).
Again in the Italian remake, the side story "Mami l'Invincibile" ("Mami the Invincible") shows that Mami Tomoe is another, with stunts ranging from shooting a rival Puella Magi with a Tiro Finaleduring a speech to ribbons that are either explosive or made of tear gas and attacking Walpurgisnacht with a magical nuke. A number of magical girls (including Kyoko) live in the fear of giving her a reason to come after them.
Enhance knows the other Dead Apostle Ancestors he's hunting are way stronger than he is, so he balances it with such things as Holy Weapons provided by the Church, a demonic sword he stole from his first kill, the fact they're Squishy Wizards and he's former military, and other tricks to come close and personal. The first battle he had on screen involved him finding out of the Broken Masquerade before most other Dead Apostles and thus using the newfound ability to interact with magicals of other kind to put together an artificial Walpurgisnacht and sicking it on his target, both to deplete his forces and get him tired and to sneak upon him and drink his blood and his powers with it.
The Games We Play has Jaune, who Had To Be Sharp seeing as he's so often pitted against far stronger foes who would destroy him utterly if he tried to fight fair. He goes for weak points, turns out the lights while using equipment that lets him see in the dark, uses his foe's fear against them, uses the Hunters' refusal to fire upon a city to his advantage, fires into the Grimm to use them as distractions, abuses Dust crystals to keep himself healed and lets his elementals distract his foes. Even as he comes into power, not only does he not get complacent, but his increased options only enable him to cheat ever more vigorously. For example, after deliberately using a showy Awesome, but Impractical move to invoke The Worf Effect on a professor, he notes in his internal monologue that if they had been really fighting for real, he would instead have used Delusory to hide himself from his foe, then overwhelmed the foe with invisible psychokinetic strikes from all directions; after all, ranged combat ultimately boils down to hitting the foe and not letting him hit back.
In The Man with No Name, Mal is quick to try just shooting the villain once things go to hell. It doesn't work, but hey, he tried.
In The Open Door, NewChaos know full well they don't have the numbers and industrial-logistical machine to match foes in a fair fight, so they do their best to avoid any universes that can pose a threat in the first place. They use their superior personnel and technology aggressively, reconnoitre in force, and deliberately downplay their abilities to lure enemies into overconfidence.
Violet becomesthis in the later episodes of Rise of the Galeforces.Then again, anyone who sics three baby T. rexes on the Big Bad in retaliation for shooting them (with extremely gory results mind you) can't exactly be considered honorable... right?
In Warmistress of Equestria (sequel to The God Empress of Ponykind), Sleight-Hoof hits Luna in the face with a bag of sand during a sparring match, in an attempt to blind and disorient her. It fails because Inertia Is a Cruel Mistress, but Luna agrees that it would be a valid tactic in combat... just not sparring.
Ulyssian's preparations for his duel with Jael in The Odyssey included, but weren't limited to: poisoning him with contact poison via his armor, breaking into his apartment to steal his money, then placing it in a trash chute to separate him from his twin (neutralizing his Gemini bloodline), and hiring a sorcerer to summon demons to ambush him.
Forward also plays with this, including replicating the legendary Raiders of the Lost Ark scene with River casually shooting an Arrogant Kung-Fu Guy who tries to fight her with a bo staff. At one point in the most recent story arc, Mal points out that he considers any fight where he's forced to fight fair as inherently unfair.
In a ShoutOut to the Indiana Jones scene, During the retaking of Mithril Hall, Johnson and Drizzt encounter a Drow who spins her scimitars around in a fancy display. Drizzt is about to go fight her, but then Johnson just shoots her.
In Harry Potter and the Methods of Rationality, Professor Quirrell explains this in the very first class of "Battle Magic", formerly known as "Defense Against the Dark Arts", that the methods that their textbooks show are useless in a real situation.
Quirrell: Your useless excuse for a third-year defense textbook will suggest to you that you expose the mountain troll to sunlight, which will freeze it in place. This, my young apprentices, is the sort of useless knowledge that you will never find on one of my exams. You do not encounter mountain trolls in open daylight! The idea that you should use sunlight to stop them is the result of foolish textbook authors trying to show off their mastery of minutia at the expense of practicality.
He later states that one should use only one of two things, if you can: either the Killing Curse or teleport away.
In Queen Of All Oni, one of Jade's Co-Dragons, Left, comes off as this. While he — like his "brother", Right — usually fights with his swords, he'll resort to quicker methods if necessary. For example, during the assault on Lung's fortress, he breaks out a chainsaw to get by a Hybrid Monster, and when confronted with a gateway that's impervious to magical attack, he simply blows it open with dynamite.
Captain Black's Number Two, Agent Wisker, also qualifies. When Jade infiltrates Section 13 to try and steal the masks in the Vault, he stands in the way. She attacks him with magic, and he retaliates by shooting her — intentionally non-lethal shots, and it doesn't even work, but it's still the thought that counts.
He also tries to (lethally) shoot Hak Foo in a later fight. Hak dodges and breaks his gun arm, but hey, he tried.
Jade's foreman Blankman — we finally get to see him in combat when he fights Drago, and while he primarily fights with savate (a French martial art), he does enhance it with magic. And when even that isn't enough to win, he resorts to whipping out a mystically-enhanced shotgun.
Pretty much everyone to some extent in Game Theory (Fan Fic), but special mention goes to Fate, during the TSAB assault on the Garden of Time. She uses ambushes, Anti-Magic Fields to weaken them and scramble their sensors, shuts off the lights to blind them, Mecha-Mooks to even the numbers, and seals the doors and rearranges the corridors to isolate small groups of enemies and prevent them from reaching their objectives.
As befits a full-scale war, A Feddie Story has pretty much nobody fight fair. Both sides are quick to employ ambushes and exploit it when they have a local superiority. Earth Federation tank crews tend to go several steps beyond Zeon mobile suit pilots, though, because they know they're at a disadvantage to start with.
The Earth Federation Air Force is more than willing to exploit the fact that they're trained to fly in all kinds of weather and Zeon's pilots (trained in climate-controlled space colonies) aren't. They also sometimes stage week-long round-the-clock bombing campaigns against Zeon ground units, not because of the casualties inflicted but because they are trying to break them psychologically: they know it makes the Zeon troops feel that they are defenseless and it denies them much chance to sleep or eat as long as the campaign goes on.
In Diaries of a Madman, Navarone doesn't just show this, but actively shows his contempt for fighting honourably on multiple occasions. Given that doing so would almost certainly have gotten him killed, he does have a point.
In The Life And Times Of A Winning Pony, Cloud Kicker notes that both Blossomforth and Fluttershy are fighting their hardest just to win one little catfight. She does note that they probably didn't learn any formal training, so fighting dirty was pretty much the only thing to do.
Bad Future Crusaders has both Apple Bloom and Scootaloo. The former calmly remarks she fights to win after being chastised for pulling a gun on an unarmed opponent, and the latter actually calls out Lightning Dust for comparing her to Rainbow Dash who didn't fight dirty.
In the Dark World timeline, once Fluttercruel gains Reality Warper powers, she instantly makes the air around the heroes turn to stone, commenting that unlike Discord, she's not going to screw around. The only reason why it failed to take them out was because Pinkie Pie's Toon Physics powered Offscreen Teleportation allowed her to escape. In the ensuing fight, almost everything she tries is a killing blow.
Prime Fluttercruel is also this trope: she's very willing to fight dirty when she actually fights. As severalChangelingsfound out during the Wedding Arc.
In The Witch of the Everfree, Nightmare Moon is impressed with Sunset's willingness to blow up Ponyville's town hall for the sake of stopping her.
Naruto: "In the fights we've had that were pure taijutsu, who won?"
Naruto: "In fights where ninjutsu is allowed who wins?"
Sasuke: "You... because you cheat."
Naruto: "Since you've gotten your Sharingan, has that changed?"
Kitsune no Ken: Fist of the Fox: Just about every fight shown has the combatants as this, in accordance with the story's direction toward realism. The best example is Shino, who gets Kiba to surrender their qualifier match in Sasuke's tournament by grabbing him in a submission hold and then threatening to break his arm if Kiba doesn't yield.
In Vapors the original character Aiko Uzumaki is one. She knocks out Zabuza because he can't believe that she'd use a lightning juts that would also hit Kakashi, she purposefully takes a hit to invoke Naruto's Roaring Rampage of Revenge on Gaara, and she uses shock and ambush tactics to humiliate a strike team of 8 jonin that include a Kage's bodyguard and a jinchuriki.
In A Teacher's Glory, Anko makes it very clear to her students that if they get into a fair fight, they've already failed.
Doing It Right This Time: In chapter 2 Misato reminds Shinji that the Angels DON'T play by tournament rules and she wants that her pilots are willing to fight dirty.
Higher Learning: Invoked and reinforced in chapter forty-eight's prologue: "Bring a bat to a fist fight. Bring a knife to a bat fight. Bring a gun to a knife fight. Run the hell away from a gun fight."
Once More with Feeling: When Shinji and Asuka sortie to fight Israfel, Asuka complains "two against one is not a fair fight". Right away Misato tells her the last thing she wants to give the Angels is a fair fight.
Kaoru from the AU fanfic Frozen Moonlight is quite willing to let her opponents think she's helpless while hiding the knife taped to her wrist or to try bashing their skulls in during their monologues. Notably, her father specifically taught her to think like this and to use other people's perceptions of her being vulnerable to her advantage.
Beelzebub from Sonic X: Dark Chaos is one of these. Rather than using a gigantic continent-sized flagship like Astorath, he uses a small battleship armed with a cloaking device so he can surprise foes.
Tsali uses his robotic abilities - super strength, super speed - and his Chaos powers - energy blasts and telekinesis - in lethal and unique combination, allowing him to fight Super Sonic one-on-one and win. And he nearly defeats both Super Sonic and Super Shadow again later on.
Sonya uses Demon-made plasma weapons against Shroud in order to burn them, as she knows they are highly resistant to ballistic projectiles. She also handily averts Katanas Are Just Better (and she even mocks the trope at one point despite it being linked to her Japanese ethnicity), only using her plasma katana as a backup for emergencies.
Tephiroth stabs Tsali in the back in Episode 72 rather than get into a long battle with him.
In Prison Island Break, since they're all convicts, most of the cast does this, even Sonic - but it's emphasised that the Knife Nut Shadow will always find a shiv even directly after a shakedown, and can make them out of anything. He constructs a shotgun out of his bed, and matches.
In Urusei Yatsura The Senior Year, during part 10, an OC gives this advice:
"Now, here's something I once heard when it comes to a fight! You kick them in the balls, stab them in the back, poke their eyes out, and if they're still in the mood to fight..." she gives them a fanatic sneer, "...THEN, you fight dirty!"
And a few paragraphs later...
Mie then transforms into a female General Patton. "Then go do it!" she points to the door with her riding crop. "Remember, you can't serve your country by dying for your country! You serve your country by making the other dumb bastard die for his country!"
The Basalt City Chronicles has Tors Beers (who hates fighting) taught to end fights as quickly as possible—biting, clawing, breaking bones, and outright killing are all acceptable means to win (though it's best to avoid the last).