In Act II, when you are inside the Pirate ship ride you will come to a cutscene where you and Didi board a roller coaster. At the end of the cutscene, a squid tentacle comes down and blocks your boat from moving forward on the track. Once you gain control of Dawn, look around for the only physical walkway up to the higher areas. As you approach and begin to climb the first ramp, look just a bit to the left and you will see a Treasure Chest opening and snapping closed its lid. Simply time your fall to land inside the chest when it is open, and this Trophy is yours.