It’s been highly speculated that Kaido’s club was made of sea stone, which would explain why Luffy took so much damage from one hit.
His Gears would have been turned off as soon as he was hit with sea stone and he must have taken full brunt force of the attack on his normal body(at his face where there was no haki), which is in line with what we saw afterwards(his Gear 4 was turned off, and his whole face was smashed). It would also explain why Luffy was so angry even though he himself picked a fight with him.
Haki vs Sea Stone?
I think that a strong Armanent Haki could give full protection against sea stone, but because Luffy leaves his face and majority of his torso without haki, he got an instant ko. I think haki can give protection against sea stone because if not then it would have been very easy to defeat a guy like Luffy, Katakuri or even Kaido, you just need to throw a sea stone bullet(or a nail) and the devil fruit user wouldn’t be able to do anything afterwards.
Why Kaido shouldn’t be strong enough to oneshot Luffy?
Because if Kaido really only used his strength in that shot, then it would be impossible for Luffy to ever defeat a guy at the level of Kaido, even if he gets a stronger Gear or any power up because Kaido didn’t even use any of his abilities in the fight(not his main Zoan transformation, not his Awakening, not Haki). He literally used nothing. And becuase of this I think that without using his main or the strongest attacks, Kaido shouldn’t be able to oneshot a guy which is at the Yonko Commander level atleast.
Main strength of Kaido?
Even though Kaido didn’t use any of his main strengths in the small fight, i can definitely say that the main strength of Kaido is his body, even without haki he withstood a jump from Sky Island, he took most of the stronger attacks of G4 directly on his chest(not the strongest). And I would assume in his strongest form and with haki, even attacks like King Kong Gun would not be able to do much (if any) damage to his body.
Why is Kaido so hard?
I think it’s a form of Tekkai and with the small encounter between him and Luffy, i was getting strong vibes of CP9 Tekkai.
How Luffy will be able to defeat Kaido?
This is pretty hard to speculate. I think Luffy could be as fast as Kaido with G4 Snakeman. But the two things that he lacks against Kaido is a much stronger body and stronger attacks. I think going by the past trends in which Luffy learns his opppnents moves to defeat them, Luffy will going to get training of Tekkai or Armanent Haki or both, and since Luffy already has a stronger body, it shouldn’t be too hard for him to gain Tekkai or make his Armanent Haki stronger. He just need someone to teach him, so I definetly think he would go under some form of short training. And this would in no way make him as strong as Kaido but it would make him able to atleast go against him with support of Kid and Law.
Role of Law and Kid
Law would easily be able to compensate for lack of speed of Luffy by transporting him for either dodging Kaido’s attack or to surprise Kaido, Kid could either support Luffy in offense or act as decoy. This would work for Kaido’s normal form but as he comes to his true form, I think at that time Luffy would need a form upgrade.
*Theory by shreejeetp