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.