To get this trophy, you need to make sure that no cadet dies on a mission. Keep an eye on you and your comrades HPs, constantly dodge and attack when given the chance, and you should be fine. The easiest way to get this would to replay the first mission alone, as you will only have to worry about the character you control and no other, so as long as you don't die, you will get the trophy.

If you end up failing, you can redo missions from the title screen and try again at any time.