The ACV is a 2 person hovercraft vehicle available only in the Naval Strike DLC.
The only way to get a kill with it is to get a roadkill. The good thing is that the ACV can glide over land as well as water.
I recommend that you play LOST ISLANDS on CARRIER ASSAULT LARGE as the map is fairly flat and it is easy to control the ACV. Keep circling until you find some poor soul stuck in the water running at half speed.... and SPLAT run his ass over.
*Note 1 - On Carrier Assault Large the ACV is available on Lost Islands, Operation Mortar, and Nansha Strike, NOT on Wave Breaker*
**Note 2 - I can confirm that getting a kill while riding as a passenger on the ACV DOES NOT get you the achievement**