Exactly, I know that the resistance of Luffy itself is one of his worst weaknesses in this form, but you have to understand that he received blows, a breath of heat that clearly causes him to reduce his resistance even further.
In the end, if he is not capable of mortally wounding dragon kaido, he will end up falling an hour or hour due to lack of resistance.
Oh unless you give him infinite resistance, kaido at least can fight 3 days in a row, luffy could take 3 days punching KAIDO without falling from exhaustion? the answer is no! That's why I can't imagine the scenery of Luffy alone beating Dragon Kaido.