This is in Chapter 16 of the Main Log, and requires you to fight Akainu. You won't get this if you're trying to go for the following trophy:
As soon as the cutscene where Akainu wounds Ace ends and you take control of Luffy, attack Akainu. When his whole body turns to lava, look around for a nearby ice block. Target it (Press R2 once you get a "lightbulb" icon) and grab it with the Square Button, then target Akainu (R2 again) and fling the ice at him to disrupt his powers, making him vulnerable to physical attacks. Repeat this until his life bar gets down to 1/3 and a cutscene triggers. Rapidly press the Square button when prompted, followed by R2 + Square.
The next part of the battle takes place on a giant hand made of magma (cool, huh?). Bring Akainu's health down to about 1/9 or 1/10 and he'll jump off the hand and try to crush Luffy with it. Rapidly tap Square to break free, then go Gear Third with Circle to counterattack, rapidly hitting Circle when the attacks collide.
Normally, you would need to stun a boss and press R1 when prompted to trigger an event attack; however, with Akainu, you don't seem to need to do that. Go figure.