Please note this is a solution to kill the Juggernaut (big fat zombie) with a vehicle.
for this you are going to need an M67 grenade & an Anti Personnel mine.
When you come across a juggernaut get out of your vehicle then place a mine about 5-10 metres in front of the car & the juggernaut should be about 20 metres away in an imaginary straight line.
now equip M67 grenade & throw it at the juggernaut.
A direct hit will rile him but he may not run after you.
now get in the car & honk your horn. This will definitely rile him & he'll come running straight at you. He'll step on the mine which will kill him. As long as you stay in the car it counts as a vehicle kill. This method was how I popped the achievement