Its stupid, but in order to kill someone on the ATV, you have to line them up with one of the wheels.
An ATV might be safer against missiles and tanks because its a smaller target, but it is slower and you're also completely exposed, the combination of which makes you extremely vulnerable to small arms fire. So not only are you less likely to actually connect with and score a kill on the ATV, you're far more likely to simply be shot off of it before you even get close enough - its gotten so ridiculously easy for me to simply dodge an ATVer trying to roadkill me that I now stand my ground in a game of chicken as I instead have some fun and try to shoot and kill the driver instead...of course this has gotten boringly predictable as well with me almost always killing the driver or even destroying the ATV.
Its easiest to get the achievement on one of the larger vehicles maps (Atacama Desert Conquest for instance) and use a jeep as they provide a much wider kill zone and they also are the fastest land vehicles in the game by far, much faster than the ATVs after they nerfed the ATV speed a couple patches ago. Naturally, the increased speed and big bumper make things far tougher for potential victims to dodge, and the enclosed vehicle and increased hit points will likely keep you alive longer assuming you make it a point to avoid tanks and engineers.
Just jump in a put the pedal to the metal and just sort of figure eight around a hotly contested area and you'll eventually get it.