Othar apparently has the catchphrase "Foul!" for whenever this sort of thing happens.
Agatha vs. her cousin:
Zola Anya Talinka Venia Zeblinkya Malfeazium: Bringing a knife to a gun fight doesn't seem very smart, now does it?
Agatha Heterodyne: Well, I suppose it isn't that much worse than bringing a gun to a clank fight.
DuPree. The best example is her fight with Zulenna.
The Jägers are all over this trope. In fact one of them expressed this twice during just one fight:
General Zog: Killink hyu enemies alvays counts.
General Zog: Iz still cheating. But in der GOOT way!
In The Order of the Stick, Haley has no problem fighting very dirty. For example, she ambushed Crystal as she was in the shower, catching her without her weapons, armor, and protective jewelry, and killed her while she was stunned on the floor. It might be argued that this is dishonorable, but aside from that being the whole point, Haley is Chaotic Good, not Chaotic Stupid.
Considering Haley's class and the above commentary on D&D rogues, this is justified.
Belkar also fits into this category, being quite willing to fight sneakily with a paladin and either taking potshots or using traps against her.
When he knocked said paladin unconscious, however, he actually waited until she woke up again instead of simply killing her. He even let her believe that she had merely become dizzy for a few seconds. He did steal and drink her healing potions while he had the chance, however.
For Belkar, this was less about being honorable and more about pissing her off. Obviously, he can't annoy her if she's out cold. Not to mention that his plan had been to let her kill him so that she lost her Paladin powers, while he could always get resurrected later.
In addition, when Belkar was confronted with the son of a Kobold he had killed, who was in full revenge mode, he was unable to kill him due to his Mark of Justice. So does he accept his fate? Hell no! He simply hires mercenaries mid-fight to slay him.
Belkar: "50 gp bonus to whoever makes him scream the loudest!"
Miko herself surpisingly pulled this off while appearing to be more of the Honor Before Reason type. When facing a group of enemies, she rouses them all to wake up, allows them to pick up their weapons and even lets them eat first, allegedly so it'll be a fair fight. In truth, this was all a ploy to get them into one group so they could all be hit with the same attack.
Belkar is an even better example in the prequel book On The Origin of PCs:
Guard: You knifed seventeen people in a tavern brawl.
Belkar: Hey, its not my fault they brought fists to a knife fight.
Guard: It wasn't a knife fight until you started stabbing people!
Vaarsuvius shows shades of this, particularly in his/her fight with Zz'dtri.
Vaarsuvius: Oh heavens! I cannot believe I violated the detailed rules and regulations we agreed upon before starting this contest! We must consult the referee for an appropriate penalty regarding my heinous transgression!
Redcloak has always been pragmatic, with examples such as using Titanium Elementals instead of Earth because they are lighter yet stronger (if distasteful, not being a "classic" element); walking his hobgoblin army into certain death because he only needs them as a distraction (prior to gaining a change of heart regarding racism); creating decoys of Xykon to make the heroes guess which one's real, but then having the real one take an entirely different method of approach; etc.
Later during the siege of Azure City, Redcloak averts this when he accepts the duel with the human cleric while he still has an overpowering army (he doesn't want to waste lives on his side).
Still later, however, he plays it strange when he is proposed to a fair fight by Thanh, who is held by an elemental summoned by Redcloak.
Redcloak: Stupid risks are just that: stupid. Crush him.
Xykon (paraphrased): ... I've found that only two things really matter. Power, in as great a quantity as you can muster, and style... and in a pinch, style can slide.
During his gladiator match against Thog, Roy uses discarded crossbow bolts as improvised shivs, smashes an empty potion bottle into Thog's face, throws sand in his eyes, and eventually wins by goading Thog into dropping the ceiling on himself.
Tagon, from Schlock Mercenary. His first pugil-stick practice with Schlock involved slicing Schlock, and his pugil stick, in half with a concealed Razor Floss wire hidden in his shoes. Don't worry, Schlock can survive that sort of thing.. His second, against the neophant Chisulo, involved lobbing the pugil stick into his opponent's eye from across the ring. The Seventy Maxims of Maximally Effective Mercenaries deals heavily in this trope; Maxim 31 even explicitly states "Only cheaters prosper".
Chisulo: I'm pretty sure that's cheating.
Tagon: I'm sorry, did you just say, "Thank you sir, may I have another?"
Chisulo: Thank you sir! Point...um...taken.
Schlock himself is pretty pragmatic.
Kathryn too, to the point that she's one of the few people to have disabled Schlock.
In Errant Story, Jon Amraphel is the only main character without any magical ability. How does he cope with fighting Magic Knight elven military, monks with Bullet TimeSuper Speed, and tiny fairy demigods? By a) being very good with guns, and b) cheating like hell. Being a semi-retired assassin, he does his best to avoid anything like a face-to-face fight.
Pinton the Pig in Future Pig is definitely this. See him teach it to a student here.
Jack Noir in Homestuck is a big fan of teleporting behind people and stabbing them in the back. Though he is omnipotent, so perhaps he doesn't count.
Winning his immortality by killing the Black Queen with the bunny does, however.
Vriska Serket and her ancestor, Marquise Mindfang have no qualms with using their psychic powers to use others to kill or maim their opponents.
Dave is not above using his friend who their foe cannot bring himself to harm as a human shield. AUTO-HARLEY!
Terezi knows that Vriska has to be stopped to preserve the timeline and the lives of all the other trolls, and that she would have no chance against her in a fair fight. So she tricks Vriska into a psychological game of chicken and stabs her in the back.