Army of Two

Choose the air carrier mission, and have an RPG instead of a sniper. As you are parachuting down, look down towards the closest crates towards the planes, there should be a group of 3/5 enemies there in a group, fire into them to kill them all, and then as soon as the rocket hits and they are dead, reload checkpoint and repeat.

Took me around 6 goes to get the achievement and i started with 0 kills.

Found this to be the best place because its right at the start of a level and theres a group of men.

*EDIT* I got this on the EU version, for anyone questioning why i dont have it.

A good place 2 work on this acvievement is while you are going for the Fission Mailed achivement on mission 2 after parachuting in and blasting martyrs stick around. Just about all the martyrs drop special weapon ammo so make sure you have the RPG equipped and when you get out of back 2 back scoop up ammo and kill some more baddies before you exit out and redo.