Easy solution, but involves a little work, is on the Face-Off level.
Do all the things you need to do to open the gate, then build the bridge, then open the next gate into the main compound. Jump in the security truck that you picked up at the beginning of the level, and drive it into the horde of henchmen that run towards you.
You should get the achievement first time, as there are so many henchmen - but if you don't, just keep turning around and going back at them. As long as five or more are lying on the ground at the same time, you'll score the achievement.