Throughout One Piece, the Straw Hats faced many powerful enemies, which they defeated either by a stroke of luck or their unique teamwork. All these adversaries are powerful, but even the most powerful have weaknesses. In the series buildup, Luffy always has a problem fighting against logia users due to their weird abilities, and the only way to overcome them is with the busoshoku haki. Crocodile was the first powerful logia user that Luffy fought, and it took Luffy to find out his natural weakness before he could be defeated. Other enemies like Lucci of Enies Lobby, Enel of Skypeia, and Katakuri of Whole Cake Island, amongst others, were all formidable enemies; unearthing their weaknesses coupled with Luffy's resilience resulted in their defeat.
The Wano arc is the Straw Hat Pirates' biggest test yet as they face one of the strongest men in One Piece, emperor of the sea, Kaido, and his beast pirates. Kaido jumped from Sky Island to see if he would die, which he did not. As a devil fruit user, the sea would have been the perfect place for him to die, but it seems the Beast pirates' emperor wants to die honorably and fight a formidable opponent like Kozuki Oden and these were the key elements that led to the Yonko's defeat.
While most of the content has been adapted already, the article includes some spoilers for the anime as it discusses things that have happened in the manga that have yet to be covered by the series.
Dragon Form
Kaido's devil fruit is the Uo Uo no Mi model: Seiryu, a mythical zoan fruit that transforms the user into a fully-fledged Azure dragon. The dragon form the Yonko transforms into at will has immense strength and an intimidating aura; an enemy would most likely feel defeated before the fight begins. While his human and hybrid forms allow him to be almost impregnable, the long-ranged attack is a monstrous feature of Kaido's dragon form, but it makes him an easy target for opponents. The Akazaya nine landed a huge hit on Kaido in his dragon form; even though the Yonko prefers to take hits, attacks that injure him deeply are primarily from this dragon form. However, this does not undermine the strength of this form, but fans of One Piece cannot overlook the bulgy scar inflicted by Kozuki Oden in Kaido's dragon form.
Overconfidence and Recklessness
Throughout the fight in Wano, Kaido usually allows his opponents hit him as many times as possible till he finally lands his death blow. Kaido's impenetrable dragon scale makes him cocky even in the face of a dangerous attack. In his first fight against Luffy, in a town close to the Kozuki's castle, the Yonko made Luffy hit him as many times as possible before finally knocking the rubber man out. In other words, an opponent who pretends to be weak but has a trick up his sleeve will most likely surprise the Yonko with a life-threatening attack. The viewers saw how Kaido's cockiness landed him a big scar on the chest, and Zoro penetrated his thick skin with his newly unleashed sword style.
Kaido believed Oden was the best of the Wano warriors and the Akazaya nine were just revenge-thirsty followers that would succumb to his empire's might, but he was wrong. The Uo Uo no Mi man's greatest mistake was underrating the Samurai-Ninja-Pirare-Mink alliance and the boiling seed of rebellion in the hearts of the people of Wano. Also, his partnership with the Big Mom's pirates further strengthened his belief that he is invincible. The famous line "pride goes before a fall" is deeply entrenched in how Luffy defeated Kaido.
Luffy’s Awakening Form: The Gear 5
Awakening is the ultimate level for every devil fruit user and a manifestation of the power they used to possess. Eiichiro Oda introduced the concept of the devil fruit awakening in the Dressrosa arc with the character of Donquixote Doflamingo. Awakening is not just about having a power boost; it is the secret to unlocking the hidden abilities of a devil fruit. In Wano, three new worst generations, Luffy, Kid, and Law, awakened their power, and viewers see the peak level form of each of the characters’ devil fruit power.
Gear 5 is the awakened form of the Hito Hito no Mi, Model: Nika, and it was the final straw Luffy used to defeat the almighty Kaido. Luffy’s devil fruit awakening gives him the power of physical transformation while he can also manifest his power to turn his surroundings into rubber. In the end, it was not just Luffy's awakening form that single-handedly wiped off Kaido in the battle. Still, the Ryuo technique and Luffy's handiness of Haki also played a vital role in the Yonko's defeat. In the battle, Luffy’s Gear 5 transformation was just Kaido's final nail in the coffin.