Stratagems that can be used for this achievement:
GR-8 Recoilless Rifle
AC-8 Autocannon
FAF-14 Spear.
Pick one to call down during a mission, or have a friend call down one. Have your friend pick up the weapon itself, and you take the backpack. Have your friend fire a shot, stand next to him and reload your friend's weapon. Achievement unlocked.