This is during Chapter 11 (Pirates VS CP9) and during Luffy's fight with Lucci, but it will be after Luffy goes Gear Second as part of the story progression (when Lucci's health bar is down to 1/3). The battle will go outside and involve several marine battleships. Beat up Lucci enough and he'll move onto one of the battleships, and that ship will fire its cannons at Luffy.
Dodging Lucci's flying kick attacks, use the Gum-Gum Balloon the the cannonball fired from the ship. Timed right, it will trigger an event; press the square button to rebound the cannonball and cause Luffy to fly towards the ship. Target the side bar and press square again, and then target the upper railing of the ship and press square to board the ship. Defeat the marines onboard to trigger another event: Lucci will attack and Luffy will escape into the air. Lucci and Luffy will start throwing fists, and you'll need to alternate pressing the X, Triangle, and Square buttons in random order until the time bar runs out. Lucci will appear behind Luffy to try and hit him with an attack; quickly hit the X button to counter.
Trophy unlocked.