This is THE hardest event attack Trophy to obtain; you can get this only in Chapter 15 in the Main Log.
Aokiji serves as the boss of the stage, and he can freeze you (even while you're in Gear Second) using Ice Trap (making a circle of ice that freezes you when you step into it). After his life goes down to 2/3, he'll start turning his body into ice. When he does this, you can't hurt him, though you won't get frozen just by touching him unless he uses the following attacks: Ice Time (he'll charge at you, leaving a small ice trail behind him that will also freeze you if you touch it), and Ice Age (he'll kneel down and touch the ground, instantly freezing a large circular area around himself and anything within the circle with it). However, when he does use Ice Time/Age, he'll turn back to normal, leaving a short opening for attacking him.
As you fight Aokiji, you need to first get him down to 1/3 of his life bar AND totally stun him, meaning he needs to be down on his knees and both a yellow and a red star circling over his head. THis is can be difficult, but try using the Square, Triangle, Triangle attack as that stuns enemies easier. You should see an indicator to press R1 while you're next to Aokiji: press it to start the event attack. Aokiji will throw a huge ice ball at you, so rapidly press Square when prompted, followed by X once to dodge Aokiji's next attack, and then rapidly hit Square again to hit him with a Gum Gum Jet Storm.
Note: if you bring Aokiji down to almost empty and haven't successfully done the event attack, just let him kill you and you'll restart the fight with him.