Here's how I got the Achievement; I'm sure there are other places you can get it too.
In the Abadis Forest, head to the room which has a long bridge with a bomb at the left end and a save point at the right end. It's two rooms below and one to the right of the top-most point on the map, and one room left of the Challenge Room.
Enter the room from the left side of the screen, just next to where the bomb grows. Get three groups of flying enemies to chase you and run back to the bomb. Use the Dust attack to pull the bomb off; it will explode upon contact with an enemy.
With any luck, you'll hit six enemies on your first shot. If not, exit from the left and try again.