Ontop of the strategies above, another approach you can do in the middle of a normal playthrough : flamethrower technically never "miss" a shot as long as you touch at least 1 zombie in a single
press.
So when you come accross a flame thrower + a good volume of zombies (Hord mode or mission), just roast 50 of them, either with the same flame or by making sure you touch at least one with each "press".