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.