Monkey D. Luffy is the protagonist of Eiichiro Oda's One Piece series. Setting sail from East Blue, Luffy aims to find the One Piece and become King of the Pirates, a goal that he is very close to achieving right now.
As with most shonen protagonists, Luffy's character is considered to be straightforward and someone who doesn't think things through, thus often being considered to be quite simple-minded. However, there are several moments when he proved that despite his easy-going personality, he's a genius after all.
10 When He Fought Crocodile In The Desert Outside Rainbase But Lost The Fight
Luffy fought against Crocodile thrice and their first clash didn't go how Luffy expected it to be. Crocodile, showing his overwhelming power, defeated Luffy in under a minute and proved the reality of the Grand Line to him.
However, little did Crocodile know that the one minute was enough for Luffy to figure out his weakness to water. Luffy's future fights against him were made much easier because of his clever deduction in their first clash.
9 When Luffy Used His Own Blood To Defeat Crocodile In Alubarna
Luffy's final fight against Crocodile was quite an intense one and it saw him use every ounce of his energy to finish the Shichibukai off. After figuring out Crocodile's weakness to water, Luffy believed he could surpass him in the fight and proved it by raining a barrage of punches on him.
As he ran out of water to counter Crocodile's ability, Luffy relied on his blood instead, which is something that not even a veteran pirate such as Crocodile considered he would do.
8 He Learned CP-9's Soru Technique After Observing Them Once In Water 7
Luffy and the Straw Hat Pirates clashed against the carpenters of Galley-La during the Water 7 arc where they were ultimately revealed to be secret government agents of CP-9. With superhuman powers, CP-9 took care of Luffy and Zoro within an instant.
Surprisingly, Luffy managed to learn their Soru technique during this beatdown by observing how they moved and used his Devil Fruit to replicate that in his body.
7 When He Created Gear Third Shortly Before The Enies Lobby Arc
Luffy gained two major power-ups during the Enies Lobby arc, the first of which was Gear Second. The stronger powerup of the two was Gear Third, which saw him channel the maximum offensive output of his Devil Fruit powers at the time.
By inflating his bones, Luffy managed to turn his body into that of a giant and land hits that were several times stronger than what he could deliver normally, showcasing genius usage of his Devil Fruit.
6 When He Trained With Rayleigh & Learned About Haki In Less Than Two Years
Luffy trained in the ways of Haki under the former right-hand man of the Pirate King, Gol D. Roger at Ruskaina. Although Rayleigh stated that two years wasn't enough time for him to learn the basics of Haki completely, Luffy managed to do it before the two years were even up.
Rayleigh left the island 6 months prior and Luffy worked on improving his skills alone in that time. By the time the time-skip was over, Luffy had a complete grasp over the basics of Haki.
5 When He Created Gear Fourth To Wrestle The Giant Animals Of Ruskaina
Luffy's training at Ruskaina was quite difficult for him as he had to fight beasts much stronger and faster than him. As his normal offensive techniques weren't enough to beat them, Luffy was forced to come up with new and stronger gear.
By inflating his muscles and using maximum Haki output to increase the elasticity of rubber, Luffy gained a tremendous power boost which was later proven to be enough to take down the likes of Doflamingo.
4 Learned Advanced Future Sight Observation Haki In Less Than A Day
Once again, Luffy showed how terrific he was at fighting when he clashed with the strongest Sweet Commander of the Big Mom Pirates, Charlotte Katakuri. Katakuri could look into the future with his Observation Haki and only a handful of people could put up a fight against him.
The only way to get around his defense was to see the future, and surprisingly, Luffy managed to figure out how to do it in less than 12 hours. Ultimately, he used Katakuri's own specialty and combined it with Snakeman to defeat him.
3 When He Mastered The Usage Of Ryou In Wano Country
After losing to Kaido in Kuri, Luffy was thrown in Udon Prison where his will was to be broken by the Beasts Pirates. Unfortunately for Kaido's group, Luffy used this place to train himself and attained the power of Ryou.
By mastering this power to an advanced degree in two weeks, Luffy gained the power to hurt the likes of the Yonko Kaido, something he failed to do before.
2 When He Figured Out How Kaido's Attacks Work In The Battle On Onigashima Rooftop
Luffy and Kaido fought for the second time on the roof of Onigashima where despite everything Luffy threw Kaido's way, he failed to hurt him significantly.
After taking several hits from Kaido's club, he came to the realization that Kaido was using Conqueror's Haki in his attacks to make them stronger and this opened up a whole new world of possibilities for him.
1 When He Mastered The Usage Of Conqueror's Haki Infusion In The Onigashima War
After realizing how Kaido's attacks work, Luffy decided to channel Haki into his own similarly and although Kaido didn't believe he could pull it off, Luffy surprised him and gave him a beating of a lifetime.
By infusing Conqueror's Haki into his attacks, Luffy managed to hurt Kaido badly and knocked him down on his back quite easily.