As one of the Yonko, Kaido is among the 4 strongest pirates alive. He’s renowned as the “Strongest Creature in the World”.
He attempted to take Whitebeard’s life, something few would dare, and subsequently clashed with Shanks. When Gekko Moriah was in his prime, Kaido fought him and emerged victorious, having killed his entire crew. Additionally, Trafalgar Law stated Kaido to be able to easily kill Donquixote Doflamingo.
His strength was also recognized by his fellow Yonko Big Mom, who could not fathom the idea of Monkey D. Luffy defeating him. Kaido in fact defeated another member of the Worst Generation, Eustass Kid, whose bounty is 470,000,000 Berries, with apparent ease.
The greatest sign of Kaido’s strength is his apparent inability to die due to his tremendous toughness and durability. He has been caught and tortured 18 times, and people as powerful as the Marines and other Yonko have attempted to execute him 40 times only to fail as Kaido survived every single attempt because every single execution weapon broke when being used on him. Most notably, Kaido has jumped from 10,000 meters from the sky onto the ground, creating a massive shockwave strong enough to sink a large ship nearby, and emerged only with a headache; it has even become a hobby for him to attempt suicide, only for it to fail.
Kaido appears to have eaten a Mythical Zoan-type Devil Fruit that allows him to turn into a serpentine dragon. In this form, Kaido possesses immense durability, even without using Busoshoku Haki.
He took multiple powerful hits from Luffy’s Gear Third while in his dragon form and transformed back into his human form after being pummeled into the ground, and proceeded to receive a barrage of Gear Fourth hits that were even more powerful, yet he felt nothing more than annoyance, received no injuries, and was able to quickly get back up and counterattack without issue. This durability has given Kaido the reputation of being unable to die.
Kaido defeated the “Fifth Emperor” with just a single blow.. how could Luffy defeat him?
In one of the interviews, Oda said that he doesn’t know how Luffy should win because Kaido is incredibily powerful: “When Kaido appeared for the first time, I said that Luffy can’t defeat such a strong character yet. And I’m yet to know how Luffy would defeat Kaido. Probably my audience will not be satisfied if the reason for defeating Kaido is just because Luffy’s punch is so strong. Luffy and I have to find a solution somehow.”