Left 4 Dead

far as i know this only works in single player i tried doing it split screen but it did not work(dont know if this will be different for others) so start up first chapter in the sacrifice the docks find a white barrel there is 2 of them near the start of this level like u go down some stairs to the left its like a min from wer u start just wait for a special infected to spawn stay near barrel as possable shot the barrel when they are near it then when they die the achiev will pop.

for me a boomer spawned i shoot the barrel then the achiev poped i dont know if this wil be different for others

update:this can be in versus mode splitscreen, changed the permissions to system link.
thanks to Will072 for info :)

update:Versus splitscreen works perfectly
Also, the explosion doesn't have to kill the SI, it works if the SI burns to death as well.
thanks to stianhk for info :)

Z3RO 37 Confirmed that SYSTEM LINK works. thanks

Can't get this to work...always try to kill boomer in system link...maybe it's the update i got? (4MB)
played offline and then played in system link..achievement unlocked! even worked with hunter as he burned to death after explosion...
by Antarael Dulac-thanks for update

As of 17 Aug 11 Pyro Riot87 confirms that it Still works in versus system link splitscreen :)

i will keep updating this solution on ppl who comments in ways that it unlocked for them so that if some ppl are having trouble unlocking it they could try other ppl methods :)

Here's how a friend and I rigged this up.

Start up The Sacrifice on 1: The Docks in a Versus match. I simply started the game and never had to mash A for dedicated servers, though some say this can help with certain achievements. Anyway! Let's get to the method.

Start up the game with yourself on the human side and another person (be it a guest or a friend) on the infected side. Once the match starts, progress through the stage by going right from the weapons and then forward until you reach the fence. Turn left from here and keep an eye out for some stairs on the left and go down them. If you move through this alleyway, you should see a tall barrel with a flame symbol on it. Once you find one, simply keep killing the infected opponent until they're playing as a boomer. Have the boomer stand on top of the canister and shoot them once with the pistol, then shooting the canister and killing the boomer with the explosion. You want to try killing the boomer with the initial explosion, it won't work if they're sitting in the flame too long.

This can be done in versus online, system link or spilt screen. I did this split screen, have your second controller stand next to a barrel , id recommend shooting the special infected a few times. The reason for this is i blew up the barrel and the infected caught on fire and died from that. As the description says kill a special infected with an EXPLODING barrel. Achievement unlocked. You can also get a few other achievements playing split screen also