Hello! I am here hoping to help you progress in your goal to achieve 100% completion!
There are a few prerequisites that come with this achievement. Now even though it says kick 20 zombies into fire that much isn't true. You can spend a load of time kicking zombies
in on easy or you can kick zombies
and pick up a flamethrower at the mid point of the level as pointed out in the video below.
From what I can tell Large Exploding zombies do not count towards the number of burned. Additionally the aggressive zombies that chase you seem to not count towards it either. Even though in the video I count 20 normal zombies burned and 6 of the other types burned and it didn't pop til I hit the 20th normal zombie.
The only reason I state the above is because my achievement unlocked on a flamethrower burst of 4 enemies after the total already being 22 before the burn making 26 zombies burned in total.
The objective is to do this all in one life. Since you are in such close range to the zombies you are going to take quite a bit of damage in between each cannoli. I suggest doing this one easy which makes the game so much easier if played normally but slightly helps in the close quarters combat aspect.
In the video I unlock the achievement well before the end of the level so if you make it to where you first run into the mafia then you should be caught up in the amount of zombies needed for this achievement.
TL:DR Watch this :