100% works, Recruit difficulty, Single player or CO-OP
Weapons: SMG I used P-90, RPG
Go onto level ''Aircraft Carrier'' and get to the section whereby you have to use the harrier to blow open a metal door. Use RPG to kill the heavy armour guy then turn left and run around the stationary missiles so your looking at the brightly lit opening.
Use your SMG to kill the enemy. Can get up-to 9 kills depending on if you team mate gets the kills.
*Secondary achievement available* L'Abattoir (Read achievement description and the rest if obvious)