Left 4 Dead 2

This can be done in versus with two controllers.

Start up a level on the sacrifice (start of chapter 1 is just fine).

Once you find a white barrel with a flaming symbol on it, spawn your 2nd controller (the special infected) next to it while you're out of sight (so the AI won't shoot the infected).

Walk back, shoot the barrel and let the special infected die by the flames.

It's possible in single player aswell, but then you have to kill the AI (by switching difficulty on expert) and lure a special infected to a barrel, but might aswell ask a friend to do a versus match if you don't have 2 controllers.

Very easily done in Mutation "Last Man on Earth"(You are the only survivor and you only face Special Infected)

Start "The Sacrifice" Chapter 1...Quickly into the scenario you will come across an exploding barrel or two...simply wait for a slow and dumb Special Infected (Spitters and Boomers are very dumb) to move close to the barrel you are watching and shoot said barrel...watch the fool burn...and presto an easy 50 GS!!!

Got this cheevo today on the first attempt...Good Luck!

Make a private Versus lobby with yourself and a guest, making your main account the Survivors and your guest account the Infected. Change the campaign to The Sacrifice and start up the match.

It may also help to switch from a Dedicated Server to a Local Server, as some of the Valve servers have been known to glitch and make getting achievements impossible. (I however did not run into this problem.)

As your guest controller, move your Infected (it doesn't matter which type) into a alleyway not to far from the starting area. To the right, behind a dumpster, there will be a large white barel with "Flammable" written on it in red, along with the CEDA symbol. Position your Infected behind this barrel.

Then with your Survivor, run ahead of the bots and shoot the barrel. Don't bother trying to kill the bots, it will take to much time in this mode, and if you run ahead of the group fast enough they won't have time to steal your thunder.

It doesn't matter if the initial blast doesn't kill your infected, if the fire caused by the explosion kills your Infected it still counts.

This worked on the second try. You may have to coax the Boomer into coming closer, but once he's near a barrel, fire away. Even if the explosion doesn't kill him outright, the ensuing fire will. Good luck!

Credit for this solution goes to Achievement Hunters/Rooster Teeth. When you see the barrel with the flammable symbol on it you have to get a special infected to go there, then blow it up (will set them on fire). You cant let it get hit by bullets or it wont count.

This achievement can also be achieved on the Single Player Campaign. Simply play through The Sacrifice campaign until you find an explosive barrel, (It is white with a flame symbol on it). Turn the difficulty to Expert once all the common infected have been killed and betray your team until you are the only one alive. Switch the difficulty back to Easy. Now you will need a special infected to kill, it is recommended you do this with a spitter or boomer as they die fast and always walk towards you. You can either wait for one to appear or continue slightly ahead to make 1 spawn. Then run back to the barrel and stand behind it. Once the special gets close shoot the barrel. Although the explosion may not kill them the fire that catches on to them will slowly kill them. The achievement will pop once it has died. Very easy 50G.