This achievement requires some skill, a little bit of luck and *good team communication. The best place to try for this achievement is by far in Scavenge matches, in my opinion. If there is a Spitter on your team and they know what they're doing, they'll be camped somewhere where they can spit at the generator / car so when someone else causes a drop near it, they have a good shot at destroying the gas can. This makes it easy to know where to lie in wait for your victim as the Jockey.
*Communication helps, but if you're the Jockey and your team isn't communicating, you can just hide near where they have to pour and wait for your Spitter to hit 'em. Then, jump on and steer 'em in.
I got it on the Mall Atrium map by hiding / spawning behind the wall that is right next to the car, running around the corner and jumping someone just as the Spitter let fly the acidic expectoration. Only had to steer the survivor a few feet into it.
*Plunk* Achievement unlocked.