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.