This trophy unlocks when you complete the mission S.S.D.D.
This is the first mission of the game. Once the mission begins, you will be Private Allen and will be showing some soldiers how to aim and throw grenades, once you finish, you are told to head to The Pit to show your skills to General Shepherd. After picking up a Desert Eagle, and switching back and forth from your primary to secondary weapon, you will be ready to begin the pit course. It is not so difficult, and you can take as long as you want. When you finish, head up the stairs, and you will have to choose your difficulty, choose whatever you want to play on, or you can try to do the pit course again. Once you head up the stairs, you are done with the mission. During the cutscene prior to Team Player, the next mission, it will unlock.
Here is a video guide.
Credit to Rafael Manguito