Set difficulty to [HARD]
Look for a [Thin the numbers] mission. (the kill this many enemies mission.)
Make sure to not shoot your Primary and don't bring any support weapons. I found this easier to do when you bring Manned Turrets and Auto-Turrets. They don't count as primaries or support weapons.
If needed throw down air-strikes, orbital bombardments and etc. But manned turrets seemed to be the way to go for this achievement to be easy.