that barely did anything.
by significant damage, i mean luffy's adv CoC punches and zoro's ashura. that's all. everything else that the supernova threw at kaido barely did anything.
bruh what? firstly red roc and red hawk did barely as much damage as the g4 barrage luffy emptied on dragon kaido. and even in those, the trick is the advanced armament not the flames.
when was it ever said that kaido is weak to flames?
all he's weak to is adv CoC. any attack which doesnt have advanced CoC is barely scratching him. attacks with adv CoA are slightly hurting him but nothing significant. he heals past those easily. im sorry but all of this flame stuff sounds like a nonsensical headcanon tbh.
Kaido admitted that Red rock hurt him and he started wondering luffy's potential ceiling luffy thinks he will potentially achieve.. thats significant.. gear 4th didn't induce such a reaction even if you assume luffy used Acoa while using gear 4th..but it wasn't nearly as affective as red rock
and mind you after Kaido turned hybrid luffy claims that his attacks aren't working meaning Acoa attacks aren't doing crap and then suddenly after hakai luffy charges and pulls a red hawk and kaido dodges it.. if it was "slightly hurting" him he wouldn't dodge it like he did against gear 4th Acoa and and Kid's robo Gipson thingi which are large scale and formidable.
from this you can deduce that the fire element luffy generates (whether spirit or science or both) while imbuing haki in it affects kaido significantly more than common Acoa punches or even gear 4th attack that are just blunt and get absorbs.
not everything should be deemed a headcanon.. but rather this is just a logical deduction based on how kaido reacts to certain things compared to others