Left 4 Dead 2

My Video on How to get this as well as Scavenge Hunt

My way of doing this:
1- Plug in a 2nd controller;
2- Create a Scavenge lobby;
3-Start the game (your profile as special infected,preferably)
4-with the 2nd controller,grab a gas can and hold it until the time runs out.
5- You'll notice that even the cronometer reaches zero,the round didn't finished and the second profile still holding the gas can;
6- hit the gas-holding player.

If you follow these steps,the achievement unlocks in about 5-10 minutes.

The best way to do it is: work as a team to keep them from making it to the generator. Then when overtime starts, simply use your special infected ability to make them drop it. I found it the easiest as the Charger.