Interesting ! Luffy could decrease the elastic property of his skin (even below normal human level) to have a skin as strong and resistant as kaido and BM
The problem with kaido (and BM) is the resistance of their skin, i think weakening it by turning it into rubber is the key. Yes they can put haki on their rubbered skin after but it would not prevent them from damage. (like when luffy took damage in gear 4)