I know this is probably a little obvious, but here is how you do it:
Press left or right on the
and press
at the same time. This causes you to perform a rush attack that covers about 1/4 - 1/5 of the screen. If you can do this and kill 2 enemies during the same rush attack, you earn progress toward this achievement. Do this 10 times and you get it. I found the easiest way to do this is during Challenge mode, but if you have two spare controllers you could line them up and farm it in just a few minutes.
* Dwaggienite noted that these have to be directional attacks and cannot just be attacks where you press X without a direction.