Load up NMPD HQ on any difficulty and skip to Rally Point Delta if you just want to get this one achievement. Alternatively, you can easily combine this with Shoot and Scoot, which requires completing the level without firing the sniper rifle. At the end of the mission, you'll be fending off waves of Phantom dropships. Simply jump on one of the rocket turrets and unload on them as they approach and take out 5. I believe a total of 7 or 8 come, so you can get them all in one go quite easily, but the achievement is cumulative so you can checkpoint restart if need be.
I've included a video below with loads of achievements. For a run-through of this mission on Heroic, nabbing the Shoot and Scoot achievement as well as killing 5 phantoms at the end, start at 12:05
. The achievements don't show for these particular two because I'd already obtained them.