This is a very easy achievement to unlock. In Chapter 4, you'll be heading to the Admiral's quarters. You can get this achievement before you enter the quarters and begin the cutscene, or you can wait until after you've enjoyed the narrative moving forward. On the wall just outside the room where the Admiral has scrawled markers all over the walls is a Buck's head mounted like a hunting trophy. Shoot it and the achievement or trophy is yours.
If you still can't find it, here's a youtube video: