Am I Missing An Eyebrow?G 25

Should come naturally while doing your 2 career runs. You need to pick up the power up that looks like a yellow rocket. It has tracking, and can take out up to 2 racers per shot (but usually only hits one). Once you hit your 20th, the achievement will pop. If you don't get this naturally, you can go into split screen/free roam with a second controller to finish it up.