Play the game through once, and you should get enough CP to unlock the Dragoon and unlimited ammo for it. Then on your second playthrough play on casual and you can tackle this trophy as well as the challenges for not crafting anything and not organizing your inventory. The Dragoon is very powerful and on casual will take out enemies in just a couple shots which makes it easy to not get hit. I got through only using one but that was just for inventory management can easily be done with none taken