Simply open all the treasure chests in the game. None of the treasure chests are missable but the final chest is located in XP Shell past the point of no return after the save point the second time you enter XP Shell where alarm goes off. It will be in the far left room. Trophy should pop soon after opening this last chest. If not just reload save before and go back and look for any chest you may have missed. Easiest ones to miss are the two in the Garvelt Bridge where you fought the boss and get sent out when fight is over. Just reenter and collect them.