A commercial in Geico's "rhetorical question" ad campaign tested the question of whether the pen is mightier than the sword. A skilled ninja shows off his sword skills, and his opponent uses a pen to sign for a package containing a taser, which he immediately uses on the ninja.
"Yes, by every means possible, we go for the win."
In Johnny Cash's "A Boy Named Sue", the title character seeks revenge on his estranged father for his awful name. Sue's had to fight his whole life through to defend himself from mockery, and has become a tough combat pragmatist as a result. When Sue finds his dad, they get into an epic brawl, and Sue describes his father as kicking like a mule and biting like a crocodile. Both Sue and his dad are examples.
The Bible: Two of the sons of Jacob, Simeon and Levi ended up slaughtering all the men of an entire city because their prince had raped their sister. How did they achieve this? By telling the men they would let the prince marry their sister if all the men of the city agreed to be circumcised. That took all the fighting men out of commission, and their conquest of the city was incredibly easy. Their father Jacob, however, hated this and told them this only made more trouble than practicality for them for them in the long run, because now all the Canaanite tribes viewed them as enemies and would turn against them.
Destroy The Godmodder: A meta example: pionoplayer and aegis have both gotten into trouble for finding and abusing loopholes in the rules to gain advantage over other players.
Lukas the Trickster in Warhammer 40,000 is the dirtiest fighter in the Space Wolves Chapter, making him the dirtiest fighter in an army of dirty fighters, and thus easily the dirtiest fighter in the whole damn Imperium. He went so far as to have one of his hearts replaced with a bomb, just to make sure he takes the other guy out with him.
And said bomb is a stasis grenade that traps those caught in the blast in a stasis field, where they can only hear Lukas' laughter for the rest of eternity. The Magnificent Bastard.
He DOES play up the Space Wolves as berzerkers just so nobody realized what bloody brilliant tacticians they really are...
The real king of this trope for the Space Marines are the Raven Guard and their descendant chapters. Where as the Space Wolves simply cheat in battle by making you think they'd fight fair, the Raven Guard were organized under the ideals of guerrilla warfare and domestic terrorism.
Gabriel Seth, Chapter Master of the Flesh Tearers Chapter. In close combat, he isn't adverse to headbutting an opponent, kneeing them in the groin or biting their throats. This is represented by him getting a free attack every time an opponent in close combat rolls a one. This ability alone makes him able to go through fifty man conscript units with ease.
And, surprisingly, the Grey Knights have a beautiful Combat Pragmatist in the form of their amazingly heroic and powerful Brotherhood Champions, who can, when fatally wounded, pour all their power into one final attack that can kill anything. Such attacks are even more potent for their unexpected arrival. Not to mention that their captains have absolutely no problem calling down Nemesis Dreadknights when faced with Daemon Princes or Orbital Bombardments on their own location to spite enemies.
Which isn't surprising when you remember there job is to stop chaos no mater what, even killing innocents and using their blood as paint if it gives them the edge.
The Orks generally avert this, being content with a brute-force strategy, but the Blood Axez clan will employ concepts like special forces, camouflage, and retreating if things go bad. Other tribes consider this Unorky, but the Blood Axes stick with what works.
The Dark Eldar are this big time. To make up for their lack of toughness or physical strength compared to the other races, they use all kinds of horrid hyper-advanced weapons that debilitate their foes and give them an edge in battles; knives and splinter rifle shards coated with virulent neurotoxins, grenades that emit hallucinogenic gas, functional Agony Beams which sap strength and will from their targets to cripple them, monofilament nets that shred their unfortunate victim as they struggle, you name it. Their tactics are developed along the same lines: raids, terror attacks and the like. Or, they'll just manipulate someone else (usually humans and Orks) into doing all the hard fighting for them.
Jes Goodwin: "They're Dark Eldar. They cheat. It's what they do."
The Craftworld Eldar, as well. They're masters of asymmetrical warfare. Most of their army dynamic is fast-moving, hard-hitting shooty troops, while their melee units, the Striking Scorpions and Howling Banshees, are more for stealth and hit-n-run tactics. And they can see into the future too, so many of their wars are won before they even start, thanks to, say, a Ranger getting a headshot on an Ork warboss or Imperial general.
The Tau are a subversion, not a played straight example, despite popular belief. They may not commit to pointless last stands and frontal human-wave tactics like the Imperium, but they do commit to other kinds of military stupidity; for example, they try to avoid sieges to an almost absurd degree, and they don't train their troops for melee combat note the Tau disdain melee combat and decry it as "barbaric", although even modern real life armies at least train their troops on how to defend themselves in hand-to-hand combat, and melee combat makes even more sense in this setting given how for their main enemies, the Orks and Tyranids, it's their main combat doctrine. Commander Farsight is pretty much the only exception to this..
Too many Dungeons & Dragons classes to name. The rogue's "Sneak Attack" ability is probably the best-known example; dealing extra damage by specifically striking vulnerable parts of the body. The Sandstorm expansion in 3.5 includes a feat for blinding opponents with sand. Stormwrack includes grappling mechanics for holding opponents underwater until they drown.
Paranoia: The main book includes a "Tips for Traitors" section with such advice as "Don't shoot at your buddy the first excuse you get. This gives him a chance to shoot back. Dumb. Wait till he's busy with something else (better yet, give him something else to keep him busy), then shoot at him."
Fiend Caste Infernals are pretty much obligated to cheat - the First Ebon Dragon Excellency flatly refuses to benefit honourable actions, while dishonourable ones are fair game. They also get combat Charms that encourage shooting the enemy from a long way away or backstabbing, and which have effects like shutting down various kinds of resource regeneration so the enemy runs out of motes and willpower before long, backed up by the ability to literally siphon away enemy luck. And then there's the option to use Black Mirror Shintai to just straightforwardly turn into a duplicate of the enemy and use their own tools against them.
In the Yu-Gi-Oh! TCG, Battlin' Boxer Rabbit Puncher gains its name from the rabbit punch; a blow to the neck or to the base of the skull which is illegal in professional boxing and other combat sports. This references how Alit was willing to cheat in his tag duel with Nistro against Yuma and Dextra.
Everybody does this in Starfleet Battles (or they lose and look stupid while doing so). Special mention goes to the WYN Cluster Defense Force, composed of a few captured ships, a few purchased pirate vessels, and mostly a load of modified freighters that pose no threat at all to a real warship...except that they only fight warships that are effectively crippled by coming through the radiation field surrounding the Cluster.
Evgeny Shvarts has a play called The Dragon. When the titular dragon is challenged to combat by the protagonist, he wants to just incinerate him first, but is reminded that there is a document he signed preventing that (the dragon claims he wrote it when he was "a na´ve, sentimental, inexperienced youth", but the threat to reveal he is afraid to fight fair is enough for the battle to happen on more even terms).
In Dead End, "Baby-Face" Martin teaches Tommy that "fair and square" is no way to win a fight.
Invoked by Ben in Death of a Salesman. "Never fight fair with a stranger, boy. You'll never get out of the jungle that way."