This is attainable on multiple levels, but I recommend Episode 1, Chapter 5
. The achievement is unlocked by finding a Red vs Blue Easter Egg.
After the first major battle, you will reach the top of the structure to the first objective. At this point, follow the wall that lines this area, until you find a small break in the wall that you can look through. If you jump on the ledge, and look down into the rocks, you will find a small chest sitting on the rocks. Shoot this chest with any weapon to unlock the achievement. View the video for more information.