I found the easiest place to get this was on the 3rd pit challenge. Start and run all the way to the bottom without hurting any of the enemies. Try to drop down the the next level everytime so you don't lose too much health.
When you get to the bottom there should be a load of the golden guys with masks. This area is perfect to just keep knocking the enemies in to each other. A good way to knock an enemy into another one is to run at him and tap the left trigger so you do a running elbow move, they fly backwards regardless of their health and I knocked one into seven other enemies doing this.
It took me a few tries as I didn't really launch any enemies in story mode at all. Just keep doing this level if the achievement doesn't unlock, and restart if you kill all the golden enemies with the masks.
*added by lord verh*
keep moving in circles and pick the ones that DO NOT run at you, the always hit you first, also don't pick guys that are blocking, so you circle like a vulture and when you see two or more enemies on 1 line doing nothing; run to them while evading guys running to you that's what I noticed. Just running at them doesn't seem to work.