This is to unlock the Firing SquadGoal outfits for Kingpin, Oni and Schererazade.
complete each characters personal operation.
Kingpin - Royal Pain
Oni - Demon and Fox
Schererazade - Dimensions of the Ninja
when all of the above have been completed you will see a new operation called Firing Squad. Complete this and you will unlock the outfit for each character.
Once you have the outfits ,equip them on each character in the loadout screen before you deploy to unlock the achievement.
Edit/update - The outfit for Oni is called Ringbearer, thank you to ME3TSPINNER for originally pointing out my mistake.