Given the sheer volume of the series' cast, there's plenty of examples of this trope to be found here.
- Luffy doesn't really have any "training" in his fighting style, he is just extremely creative with his rubber powers and makes up techniques on the spot.
- Luffy's victory over the Gorgon Sisters falls into this. The sisters' mastery of Haki allows them to predict Luffy's movements and deflect his attacks. Once Luffy goes into his Gear Second power-up, he's able to move so fast that Sandersonia can't attack or dodge even though she can still predict his movements, while his attacks become powerful enough to overwhelm Marigold's defense.
- At that point, Luffy has the same level of Haki that Hancock does, but can't control it and can't do anything more than render people without sufficient force of will unconscious. Luffy's shown occasional flashes of other types of Haki use (against Mihawk, he avoided losing a hand because he saw a flash of what Mihawk's next attack would be), but they too were uncontrolled and he didn't even seem to realize what had happened. After training with Silvers Rayleigh, this has now changed as he's become both skilled and strong.
- Zoro was also this to an extent in the earlier parts of the storyline, his idea of sword fighting being "Swing your swords at your opponent really, really hard until they stop moving". With his absurd Charles Atlas Superpower, this worked pretty well...until he faced off against Dracule Mihawk, the greatest swordsman in the world. He kept Zoro at bay with a blade smaller than a pocketknife, and the only reason Zoro survived the fight was that he had enough willpower and spirit to earn Mihawk's respect. Zoro started to focus more on technique from then onwards, and began in earnest when he fought a guy with steel-hard skin who was basically immune to brute force.
- Mr. 5 is a case where the "unskilled" part is to his detriment: While his Bomb-Bomb Fruit ability is powerful, he's not very creative with how he uses it, which is why he's the lowest-ranked of all the Officer Agents. Compare this to Mr. 3, who ranks higher because of how he utilizes his powers despite being physically weaker.
- Blueno is this when he fights Luffy. His Paramecia is one of the strongest, and his Doriki is still only below the three heaviest hitters. But Lucci states outright his skills have dulled out due to his cover not requiring much physical work, and he is outmatched when Luffy's transformations surpass his own brute force.
- Played With for Fleet Admiral Akainu/Sakazuki. Most of his techniques boil down to either 'spam magma fists' or 'turn fist into magma and punch until target is dead', but his raw strength and the fact he's made of magma are enough for him to obliterate all but the strongest of foes in a few hits. However, he demonstrates some martial arts skills in his battle with Whitebeard in the anime, and is also capable of performing Haki and combining it with his magma powers.
- Of the Post-Time Skip villains, Hody Jones gets accused of this by Jimbei, who effortlessly blocks a powerful attack from him. He's also proven to be absolutely no match for Luffy whatsoever when they finally begin to throw down. Rather than making any effort to improve his technique, Hody instead artificially boosts his strength with Energy Steroids, which still isn't enough.
- This apparently tends to apply to Logia users. In the Grand Line and weaker seas, very few people can use Haki so most Logia users easily No-Sell every attack. In the New World however, Haki is apparently fairly common so "Logia users convinced of their invincibility are the first to go." This is shown with Caribou, who is a murderous Serial Killer with the power to transform his body into a living swamp. He can draw people and objects into his body, smother opponents, and shrug off any physical hit by turning into liquid mud, but he demonstrates no fighting skill outside of his Fruit. Whenever he fights someone who can actually punch him, he goes down like a chump.
- Caesar Clown is an interesting combination of this and Glass Cannon; his Gas-Gas powers give him a plethora of mean abilities, such as sucking away oxygen, poisoning people, and causing explosions. However, he's a scientist, not a fighter, so in battle his first and only responses are to use any one of those skills to overwhelm his opponent. When up against a Lightning Bruiser like Luffy who is familiar enough with his abilities, he goes down pretty quickly.
- Bobby Funk the assassin is enormous, incredibly strong, and so naturally tough that an Armament Haki-infused axe shatters when it strikes his back. However, he's a pacifist and a bit of a wimp, so he has absolutely no fighting technique whatsoever. This makes him the perfect target for his brother's Jacket-Jacket Devil Fruit, letting the Weak, but Skilled Kelly Funk assimilate Bobby's raw physical might and become Strong and Skilled.
- The Donquixote Pirates have two officers that are this: Machvise likes to crush people with his weight, while Dellinger blitzes his opponents with his speed. Machvise ends up being punched into the atmosphere when the giant warrior Hajrudin summons enough strength to match his force, and Dellinger goes down in one hit when up against the impossible speed of Hakuba.
- Kyros has no background in combat training, having grown up as a Street Urchin around Dressrosa, yet was able to curb-stomp the soldiers and guards sent in to arrest him. This display of talent attracted the attention of the king of Dressrosa, who offered him a job as a fighter in the Colosseum. Kyros then proceeded to have an undefeated career in the Colosseum, with increasingly strong opponents due to his ever-growing reputation and celebrity status, until he amassed an impressive streak of 3,000 consecutive victories over nine years. He only ever suffered one injury during his final match, when he was caught by surprise upon learning who his opponent was under their mask. Beyond that, during an ambush set by Donquixote Doflamingo, Kyros willingly cut off his own leg in an attempt to kill the Warlord, only to be subsequently turned into a toy while he was distracted. Kyros did not let this stop him though — even as a toy soldier, he rebuilt his image as a fearsome fighter, and until Luffy and the Straw Hat Pirates arrived on Dressrosa, was the single biggest source of problems for Doflamingo and his criminal network. The second his body is restored, Kyros immediately sets out to finish what he started. When he faces off against Diamante, while the latter's tricks and dirty tactics give him some amount of trouble Kyros simply pushes through with nothing but sheer force of will until his final attack breaks Diamante's sword as the latter is cut down.
- Jack "The Drought" of the Animal Kingdom Pirates, unlike his fellow Lead Performers. Whereas King "The Wildfire" mixes a pragmatic mindset and pyrokinesis with his Pteranodon powers, and Queen "The Plague" is a genius in chemical warfare and cybernetics alongside his Brachiosaurus abilities, Jack shows no other strategy than crushing his opponents with sheer force. He either smashes them in his Mammoth form or tries to carve them up with his blades. Tellingly, he is the only Lead Performer without named attacks.
- Big Mom, one of the Four Emperors, was born Charlotte Linlin, a natural force of destruction who was taking down full-grown giant warriors at the age of 6 by complete accident. She never had to train for any amount of her freakish might, and only got even more dangerous after gaining a Devil Fruit power that gave her the ability to snatch somebody's soul right out of their body. As a result, in the present day she has no need for much technique or skill at all, as her raw power is more than enough; even after all these years she still swings swords like baseball bats and doesn't really punch so much as she swats things like flies, but that's more than enough.
- Most Holy Knights, Sommers for example, have great abilities. Not only do they have strong Devil Fruits, but they can easily regenerate from anything that isn't Conqueror's Haki, with even basic Armament not being enough. But they are still Celestial Dragons, and their regenerative abilities don't help their complacency. Their victories are mostly against those who are either weaker than them (the majority of characters) or have an overwhelming disadvantage. Any time they fight someone truly strong in a fair fight, they are outmatched.