This trophy can be obtained quite easily on the level Salamun Bridge.
After the first section of the level you will be asked to go down under the bridge. I would recommend having an M82 Assault Rifle equipped before you drop down.
As soon as you drop down off the main bridge, look across to the other side of the bridge and you should see 3 enemies huddled together, a little lower than you and reasonably far away. Zoom in, blast away and the trophy should be yours. Like the solution above, best attempted on easy.