In the mission "Aftermath" there comes a section where you have to turn the power on and wait. You'll be defending yourself against a few waves of enemies. In the middle of the room, on the left and right side will be spinning spikes and chains. Just pick up an enemy and toss it into any of the moving parts to instantly kill it. This is a great section to get the 10 environmental kills as well.
You can get this at an earlier section, this is probably the easiest spot for it since you'll be defending yourself, throwing them into the environment is an instant kill so it makes the defend section a breeze.