You get the specs for the Racer after you meet Moxxi for the first time and Lancers drive around the highways of T-Bone Junction a lot but they have a lot of health so an easy way is to wait until you have the specs for the Lancer which is given to you after you complete the mission 'OMG APC'
.If you have an imported character who has completed the mission and has the specs for the Lancer you can load up that save and do the same below method.
Once you have both specs to spawn head to any Catch A Ride station and spawn a Lancer, then press
to switch over to the second spawn slot and spawn a Racer and get in the driving seat. Now blast away at the Lancer and once destroyed the achievement is yours.
Here's a video