Secret Service

Use the 9th level in the game. The one where you use the chopper and infrared goggles to shoot the bad guys. The machinegun has a wide range. Just wait untill you see 4-5 guys bunch together and pull the trigger. Just spray the bullets and you'll get this achievement in no time.

I found this best to do on the Extraction level while you are the gunner in Marine One. The cannon has a great area of effect on it, just keep firing into the masses of troops on the ground and eventually you'll get this to pop. If it doesn't just reload from the checkpoint and try it again.