User blog comment:Kaido King of the Beasts/Chapter 879: Mirror Battle in Mirro-World/@comment-24850639-20170922150027/@comment-2001:8003:1172:2900:A826:202C:FB8B:7679-20170924111934

Haki Gear 3 he used on Big Mom, Luffy is certainly being serious and base Luffy could at least damage Doflamingo he hasn't put a scratch on Katakuri.

People are forgetting Cracker a dude who accordingly weaker than Katakuri was perfectly able to overpower Gear 4 with his Devil Fruit and actually cut into Luffy's G4 arm. Which means Katakuri would have no trouble againist Gear 4, Luffy needs help.