There are two very easy ways to get this. On "Delta Halo" you will get a checkpoint, then come to a jackal with a sniper standing above a large clearing that you can assassinate. Do that, reload the checkpoint and do it six more times. Even though you reload the checkpoint, it counts them.
The other spot is when you come to a large open room (I forget the level) and there are flying enemies attacking some Covenant across the way. If you have a sniper, simply snipe seven of the flying things for the achievement. Leave the Covenant alive so the flying dudes stay attacking them and don't look your way.
Another spot is on "High Charity" when the drones are fighting the flood. Kill the drones during this section.
Note: "From behind" does not mean you need to melee them, nor does it mean you have to hit them in the back. You can snipe seven enemies in the face and still get this.