Killer has barrier/repel or whatever. Oda didnt even say Killer lacks barrier/repel... or else you'd have to agree Zoro also lacks it.
Both did nothing to Kaido except feel pain
"But he failed to cut Kaido's neck" so did Raizou and the rest. Even Oden wanted to cut his head but ended up cutting the side
Kidd relies on his force
The only thing confirmed is that nobody has Penetration on the roof except Luffy.
Zoro doesnt have it, Kid doesn't, Killer doesn't, law doesn't but has his hax skills.
Like I said prior, kidd issue was never ryuo but more power.
"How will they hurt Kaido' they already did
If you mean real damage? Then ask how will zoro also hurt kaido...
So far nobody made kaido bleed on that roof except Luffy