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.
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 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.
NewChaos in The Open Door always sends out superior forces to recon in force other universes, feeds enemies false information to make them fall into traps and predictable behaviour, and refuses to stick around for straight fights against factions that can be genuine threats.
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, 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.
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.
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 the final confrontation of Evangelion 2.50: You Will (Not) Survive, the Original Character Nephilim 'Gabriel' exploits Shinji's mental fragility and his own transforming powers by pretending to be a variety of people close to Shinji, breaking his will and allowing Gabriel to attack. When Misato tries to intervene with a gun, Gabriel evades her until she runs out of ammo, tries to stab her in the foot to trip her and otherwise plays against the rules. A given, considering Gabriel is an Angel, doesn't care for human rules, and is a known manipulator.
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. Eric takes this to its logical conclusion by producing a flamethrower from out of thin air.
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.
Keleria in Children Of The Stars, messy kills when available, groin attacks, biting, she gets progressively worse the more she slips into that blood rage thing.
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).
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!"