Load up the level ONI Alpha Site at Rally Point Charlie. You will start with a few marines nearby, two who should have shotguns. Edit: Turns out the weapon spawns are a bit random, if nobody has a shotgun, reload the level. Swap out one of your weapons for the shotgun and hold on to it for awhile. You'll need to fight off a few waves of enemies coming first from the stairs to the right and left of the turret, then from behind. Clear out everything that comes at you until you need to take an elevator up to the roof. On the roof you will have more enemies to fight, this is when you will see Jetpack Brutes mixed in with a few regular ones. Kill 3 of them with the shotgun you took from your ally and enjoy your achievement!