If you have a second controller or someone online to play with, you can get this one easily. Just do nothing and either take out all the enemies in the first level with controller 2, or let your coop partner kill all enemies.
If you don't want to do it with a second person, you can play through until you get Dash Explosion and then on the next level kill everything by just dashing. Obviously harder to use this method, but it will work if you can survive.