No episode of One Piece features Luffy using "Supreme King Haki" because the term is non-canonical. However, the technique most fans intend—the infusion of Conqueror's Haki into physical attacks—first appears in as a single, instinctive blow, and is first mastered and sustained in Episode 1074 .
On a distant cliff edge, Monkey D. Luffy stood panting. His chest heaved, his arms heavy with the fatigue of battle. He watched as Kaido prepared another blast of his devastating heat breath, aimed directly at the alliance’s command post. which episode luffy use supreme haki