Kaido just has insane durability/endurance and recovery speed. He is taking damage since the scabbard battle, but is still back like it's no worse to wear. He looked the most damaged after kong gatling though.
I'd say the SN damage ratio comparatively to each other look this way
Luffy - 92 %
Zoro - 3%
Killer - 3%
Law - 2%
Luffy did the most damage thus far due to being the one the most on the offense with his level3 hardened G4 punches, with Kong Gatling hurting Kaido the most(with Kaido temporarily down, and Big Mom worried)
Zoro did enough damage for Kaido to spout Luffy out, but Killer momentarily having Kaido down due to internal sonic attack, and Law giving slightly less damage with his gamma knife
Kid did the least thus far. Hope that changes though