Easy diamond crystals:
Open a chest near the beginning of a chapter. If it does not contain a diamond or other desired crystal, do not press X, as it will trigger the game to auto-save. Instead, hold the PS button, and restart the game.

You will start at the beginning of the chapter, and the chest you had just opened will be closed again. Open the chest again. If you do not get a diamond or other desired crystal again, repeat the process until you get a diamond or other desired crystal.
Easy “Barrage” trophy:
At the very end of Chapter 10-2, you will be standing in front of a large stone door after a cutscene. You will have to use a lever to open it. Just before reaching the lever, you will have to fight a group of eight Ogers. Make sure you have a full Super Move before beginning the fight. If your Super Move is not fully powered up, let the enemies kill you, and destroy the sunstones in this area again. As soon as the fight begins with the eight Ogers, use your Super Move by pressing Circle(2). The blast should kill all eight enemies, and you will get the “Barrage” trophy. If this does not work the first time, let the enemies kill you to respawn before the fight, and try again.