He can effectively trap the guy, destroy the part where he is. That's easy if you understand his power, that's what zoro did, he understood his power than destroy the upper part to force him to stay in the air and get out of the rocks, once he did he was a one shot material so basically a fodder.
And he can't hurt luffy so there is absolutely no reason for luffy to turn into G4.
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And he broke all of theirs too. Haki defense is inconsistent. And he has the best coa amongst the straw hat, like it or not.