This achievement is story-related and cannot be missed. You will get it for completing the Deadshot side-quest.
At certain points in Arkham city, you will hear a shot and receive a message, telling you to search the area. Do so to find a victim of Deadshot. Use your detective view to observe the crime scene and follow the leads to the killer.
Hardest part for me was to trigger the shots to happen. Look at the video/picture below to see the locations.
(video credit: Batman Arkham Videos)
Crime scene 1:
Crime scene 2:
Crime scene 3:
Deadshot stash:
(picture credits :
http://www.gamesradar.com/batman-arkham-city-shot-dark-side-... )
Hope this helps you guys!
Any improvements? Please let me know in the comments below