To get this Achievement in the campaign you have to eliminate a Scout Trooper with Iden's melee takedown. You can always restart the checkpoint if you need to attempt it again. Then take it to the woods where there will be lots of enemies spawing close together and look for an opportunities to kill five of them with a total of three shots. To equip the 'Barrage' just pick it up and press the D-Pad Left. You will find the 'Barrage' in a weapon box inside the bunker guarded by some enemies you see after completing the main objectives in the starting area. To get this Achievement in the campaign you must eliminate five enemies using the 'Barrage'.
