The description here, 'Stun a vehicle with an overcharged plasma pistol and quickly kill the driver'. This isn't too hard, simply approach a vehicle like a Chopper or Ghost with a charged plasma pistol and release the shot into it. The shot will disable the vehicle for a few seconds, enough time to bludgeon the helpless driver to his demise.

I got this another way though, and you may find it easier to do if your having trouble hitting a moving vehicle. I simply got one of those energy sappers(You know, the blue ball-like device that drains shields and disables vehicles), disabled a vehicle with it(You know, throw it in the vehicles direction), and unload on the driver, it unlocked the achievement for me.

Good luck. :)