On one of the levels a little bit into the game (third or fourth I think), you'll find that you're being chased by a bunch of smaller enemies who home in on you. Don't shoot them! Start circling around and let their numbers build up while also staying as close as you can to clusters of other enemies.
It's all a matter of judgement, but when you think you've got enough critters around you, let loose with your smart bomb and the achievement should unlock.
As long as you're focused on getting the achievement it's not actually all that hard to do.