This trophy unlocks once you complete the mission Team Player.
This is the second mission of the game. After the cutscene, you will be injured on the ground, General Shepherd will help you up. You will have to first shoot at the pathway across the river, or just wait. Once the enemies on the broken bridge appear, begin taking them out, failure to do this will lead to mission failure. Again, shoot at the enemies, or wait. Eventually, the vehicle will come, get in and you will be put on the mounted turret up top. After a massive explosion to a building, you will continue through to the streets, make sure you do not kill any civilians along the way. Soon, you will come to the point where you will have to shoot, so just spam at the enemies. Eventually, your vehicle explodes, and you will have to run inside the house. Progress up the stairs, clearing out any enemies and following the instructions on throwing a flashbang in there. Once clear, head outside into the school, where now, you have to go through it killing each enemy, I cannot really explain it in further detail, simply follow the linear path. When you make it outside once again, make your way down the alley toward some fleeing enemies. Gun them down and go to a narrow pathway leading to General Shepherd, this ends the mission. During the cutscene, prior to Cliffhanger, the next mission, the trophy will unlock.
Here is a video guide.
Credit to Rafael Manguito