When you first start Crisis Core Reunion, you will have the option to choose between Normal or Hard difficulty. Hard Mode isn’t really any different than Normal, except enemies will have more HP on Hard. None of the other stats are different, so enemies will just take longer to defeat. In order to get this trophy/achievement, you just need to complete the game on Hard difficulty. While you might think to do an entire playthrough on Hard, it’s not needed, as there is a trick you can do. All you have to do is change the difficulty right before the final boss to Hard, defeat it, and you will unlock this.