World War Z

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.

The Guide
I tried this numerous times on various levels and was convinced it was buggy as it just wouldn't pop for me. Then I found a single comment that suggested it wasn't buggy, just far more specific than the description implies.
Load up Episode 1 - Chapter 3 (Hell and High Water) in an offline game on Easy and start by killing the bots. Run to one of the gasoline puddles with enough zombies following you, set it on fire and stay standing in the puddle until enough have been killed by it.

Credit where it's due
Credit for figuring this out goes exclusively to a gamer called Heblon who posted a comment on this site...
https://www.playstationtrophies.org/game/world-war-z/trophy/...
Here's the link to their own YouTube video showing exactly how it's done...

Some Speculation - That you absolutely do not need to read which is why it's at the end!
What I suspect is happening here (and the reason everyone believes it to be bugged) is that the game is only registering the kill for those zombies that die either on contact with the puddle or actually in the puddle after a couple of seconds. If this is the case then the ones who run through, catch fire, stagger out and collapse don't count. Neither do the ones who run through, catch fire and then attack you while on fire before burning to (un)death. This would also lend credence to why some people report increased success rates doing it in a massive swarm such as Episode 4 - Chapter 2 (Final Call), simply because more zombies equals more chances for the specific criteria to be met...
Or I could simply be seeing a false positive because this worked for me first try after so many failed attempts and it is in fact, buggy as hell and it was nothing but blind luck that Heblon's method worked . I don't think it was blind luck but I obviously have no way to prove it one way or another.

I tried my best to obtain this achievement trying the other solutions suggesting using the map (Hell and High Water), but kept coming up short. Short of zombies to be exact. Just couldn’t get enough zombies in the fire before the fuel burnt out. So I tried another chapter, and got it first try.

First off, make sure do this offline with the A.I. teammates. Load up the map Jerusalem and choose the first chapter (Brain Surgery). Also make sure you choose the medic class and Insane difficulty. The reason for Insane difficultly, is to easily kill your A.I. partners as we don’t want them killing any zombies before the fire kills them. At the beginning as you walk through the gate, go left and you should notice, if not one, two fuel barrels. This is where you will get the achievement. Kill off all the zombies lurking in the area, (trying your best to not take damage) and make sure pick up an equipment bag as well. If you have the medic class fully upgraded, this will grant you an extra two stim charges which we will need.

Now that all the zombies that were lurking in the area are dead and you picked up your equipment bag, kill your A.I. teammates, and run up the steps to trigger the objective. The objective will cause an alarm to set off and call in a swarm of Zombies. Stim yourself quickly and run to the nearest fuel barrel. When you see the zombies start to spawn, set the fuel barrel ablaze and stand in the fire (and stay in the fire) and wait for the zombies to come running at you. The fire will begin to kill the zombies. Keep triggering your stim on yourself as your life gets low to keep you alive as long as possible. You can also heal yourself with your medkit as medics start with a medkit in their inventory. I recommend you equip the perk that allows you to apply medkits 50% faster. The perk is called ‘Triage’ The achievement should pop easily after following this guide.

Hope this helps you with this somewhat annoying achievement as it did me.

I was able to get this offline on New York Chapter 2 - Tunnel vision. Play on easy, you wont need to kill your bots. At the end of the level, you need to protect the train from a long wave of swarms/hordes of zombies. Before you throw the switch to start the battle, look down to the left of the train and you should see barrel with oil coming out. This is the barrel you will use to get the achievement.

Now throw the switch and keep an eye out as to where the zombies are rushing from. Eventually swarms will come running down this side of the station right over the oil slick. Shoot the slick and you set dozens upon dozens of zombies on fire. Some may get picked off by your bot teammates, but there are so many zombies rushing that there will be plenty left who burn alive from the fire. And since you're playing on easy, the die pretty fast. I even had an auto turret set up on this lane and I still got the achievement.