A simple way to get this achievement, is to use the mutation "Flu Season." The mutation will spawn many spitters and boomers. You will need to use the campaign "The Sacrifice" as it is the only map that features these exploding barrels. Load up chapter one of The Sacrifice with the flu season mutation. You will not need to progress far into the campaign before you come across your first exploding barrel. The chances are, you are near a boomer or spitter. Simply shoot the barrels and you should get the achievement. If, for some reason there are no special infected around, you should call a vote to restart the campaign, and try again.