The achievement is a little harder then it sounds you have to ignite 20 Zombies in one go starting a new game or loading an old save resets the counter back to 0 and headcrabs DO NOT COUNT
Also all you have to do is light them on fire it don't have to kill them for it to count.
Take a flair ignite 1 or 2 zombies then drop flare and kill with gun to get them out of the way rinse and repeat.