However, there have been many instances in which Armament Haki was unequivocally used—Boo against Kelly Funk, Luffy against Caesar Clown, and more—but remained completely invisible.
Luffy intervenes, and the two clash using their Haki. Show more Law uses Countershock to deliver the final blow to Doflamingo, but Doflamingo uses his powers to revive himself and counterattack Law.
During his training, Luffy encountered beasts (similar to what he encountered on Elbaph) and learned to control his Haki by going through ... Line (especially the New World).