As one of the Yonko, Kaido is among the strongest pirates alive, renowned as the “Strongest Creature in the World”.
We still don’t know if he uses haki or he if he has a devil fruit. We just know that he has created an army of over 500 Artificial Zoan-type Devil Fruit users, his Gifters.
Befitting his bulk, Kaido has tremendous physical strength, and is capable of swinging his weapon hard enough to send a normal-sized person flying into the distance in one blow.
Perhaps 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.
In one of the interviews, Oda said that he doesn’t know how Luffy should win because Kaido is incredibily powerful: “When Kaidou 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 Kaidou. Probably my audience will not be satisfied if the reason for defeating Kaidou is just because Luffy’s punch is so strong. Luffy and I have to find a solution somehow.”