You can get this achievement in episode 1 mission 3 Hell and High Water right at the beginning of the mission.
I suggest you to play offline on hard and kill the bots at the beginning because if they kill the burning zombies or set the gasoline puddle on fire it won't count towards the achievement.
After all the bots are down go ahead an walk downstairs until to the end of the road.
At the end of the road shoot once (without a silencer) to attract more zombies and turn right.
Walk down the alley and if you are luckey there should be a gasoline puddle right in front of you. Shoot it while you walk by to burn all the zombies behind you.
If you are not lucky and there is no gasoline puddle you can either restart the mission or walk up the alley with all the zombies in your back were you came from and find the next available gasoline puddle between all the cars.
Here is a short video if you still have trouble getting this achievement:
Note: I died at the end but still got the achievement.