Luffy learned Ryuo, a trick to flow Haki out of his body and wreck stuff from the inside. He practiced on trees and collars, then used it to hurt Kaido—something his regular punches couldn’t do.

Devil Fruits are the cornerstone of the One Piece power system. Although different aspects would eventually be introduced ...

The Straw Hats would most likely hinder Luffy at Marineford due to the chaotic war nature. Even if they took out smaller Navy units, the Straw Hats wouldn't change the outcome at Marineford.