By completing all levels (of one difficulty, can be any difficulty) with a total ranking of Silver or greater, when you reach the credits, you'll be taken to a unique minigame. Destroying the airships will grant you access to special game art and you will encounter the two hidden foes. Defeating both will add them to your bestiary. By destroying ALL airships and the boss, you will get a Trophy. Should you fail, you can simply retry by selecting to view the credits in the Map menu.