This is an extension of Colonel Skills excellent description, but its a valid point i found out that helped me get the achievement after many many tries!
When you see the last 3 spotlights before the 2 turrets, take out the top left and right spotlights first, leaving the bottom left spotlight to continue scanning the area.
Now if you watch the path of the spotlight's light on the monorail track you can gauge how close you can get before it becomes necessary to shoot it out. i found i was about 1 monorail cars length away before i had to shoot it out.
Now that you are that much closer to the turrets starting point, when you trigger AR mode when you are zoomed in with the sniper, the front cover rail in the monorail car wont get in the way when you jump backwards or forwards into AR mode.
Hope this helps you guys that nearly smashed up many controllers like i nearly did!
Edit: Thanks to a helpful comment from DL Tsu, there is apparently 3 turrets on Hard and/or God mode, i have not got that far in the game yet on those difficulties, but by all accounts, its hard! lol!