Left 4 Dead 2

With the new update that was released on 4/20/10 this achievement can be earned in Scavenge mode by yourself as long as you have 2 controllers. With the new update there can now be more than 1 special infected. Here's how I did it:

IMPORTANT NOTE: once you spawn the Jockey at the start of the match and if you don't get the achievement on the first try just hit rematch on both controllers and you'll spawn they jockey every time at least that's what happened for me.

Start up a scavenge game on Dead Center in the atrium. main account as infected, 2nd controller as a human. Restart until you spawn as the jockey, set up your jockey behind the first pillar, a little ways before the lone gas can on the first floor. Then as the survivor go grab that gas can then go wait towards the left rear of the car, kind of by a pillar as well. Keep an eye out for the Spitter with your jockey, they will be highlighted in blue kind of helps to have full subtitles on. When the Spitter spits and if it hits the survivor move him out of the goo then pounce with the jockey and steer him into it. If the survivor is already in the goo the achievement won't pop. Might be easier to pounce one of the other survivors because they automatically move out of the goo. You may have to wait for a 2nd or 3rd Spitter if the other survivors shoot her before she gets a chance to spit.

Team Coordination is a real help getting this achievement; if you let the Spitter on your team know that you're trying to get it, they may be willing to wait until you grab someone to attack. If there is no Spitter on your team, you can just wait for one of your teammates to die and become one before you spawn in.

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.

Coordinating with your team will certainly help and I've been one to do that while playing both the Jockey and the Spitter.
It's not a total necessity though as long as you pay attention to each other.

As a Jockey, watch where your Spitter is and when you see the acid patch, jump someone and steer them into it. When I first got this achievement it DID NOT pop up on my screen but after checking my achievements later on, I did actually receive it.

Also, if you're a Spitter, lend your fellow Jockey a hand, even if they don't ask for it, and send some acid their way!