There are actually several great spots to unlock this trophy, however the easiest place to unlock is at the beginning of Chapter 6: The Pit. Difficulty for this trophy does not matter.
At the start of the chapter, you are in cover behind a car, with only a Desert Eagle sidearm. Go to the right behind additional cover, and you will find a shotgun, and box of grenades.
Avoid the soldier, who may come around the far corner, and while in cover move toward the end where he came from. There should be at least four soldiers here in which you can quickly takeout with your shotgun.
Depending on the difficulty you are playing, if you did not get it, reload the checkpoint and try again. Playing on easy (Walk on the Beach) will make this a lot easier.
Below is a video for help. All credit goes to PowerPyx for the video.
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