In order to unlock this Trophy, you'll need to first beat the game to obtain the normal ending. Afterwards, you'll receive a notice about another path and will be told to replay chapter 4-3 after you have unlocked more allies. This requires playing the Crew Episodes as you'll get new allies after completing each episode, specifically Luffy, Marco, Kuzan/Aokiji, Hancock, Ace, Kuma, Mihawk, Crocodile, Buggy, Garp, Magellan, Jozu, and Vista.
Once you have met this requirement, go back to Chapter 4-2 and play through until Whitebeard is possessed by the dark dial. DO NOT FIGHT HIM! What needs to be done is Marco and Ace need to get to Whitebeard so they can snap him out of the dial's effects. A different cutscene will occur and then you can finish the chapter. Doing so will unlock Chapter 5, which has 2 episodes. Chapter 5-2 is similar to 4-3, but played a lot differently and you'll have to fight Akainu and Moriah before you reach Blackbeard.
A minor note: you will unlock Law as a playable character when you unlock the Final Chapter. If you play as him in Chapter 5-1 and fight Kizaru, you'll unlock a movie for the gallery.