Back 4 Blood

Simply cause a fire (Red barrel, Gasoline canister, Molotov), stand in the fire and kill a Ridden with the Flamethrower.

Flamethrowers can be found in locked orange Weapon Crates that require a Tool Kit to open, like the Minigun crates.
They can also spawn randomly in the levels. I found a flamethrower on a dead body, to the right side of the Saferoom door, at the very start of the level 'The Waterfront' (Act 6 Chapter 4).

[ Guide]

Flamethrowers spawn in random locations, and are not always easy to find. There is however, a chance that it spawns in a saferoom. This will occur when the corruption card 'Razor Worms' is active. That's the card where the ground is covered in thorny bushes that damage you when you run through them.

In my opinion, the best way to get this achievement is to trigger that card.

How?

1. Put your game on 'No Hope' difficulty. Why? Because No Hope has more corruption cards and this increases our chances of getting the right card
2. Load up any act that you like. I went with 6-1 [ No Sanctuary)
3. Create games untill you see the Razor Worms corruption card. If it's not there quit out immediately. Do not even bother selecting a character.
4. Once you have the card., select any character (one with a lot of health is better) & pick up the flamethrower. It should be in the saferoom with you.
5. Buy molotovs.
6. Simply walk out of the saferoom. Throw a molotov at your own feet and torch whatever ridden is closest to the boat.


Notes:
- Even though there is not much else to it. Here a some additional pointers:
- You drop the flamethrower if you swap weapons. Don't be alarmed if it's gone. It's on the floor.
- Having the pyro perk increases your fire damage by 100%
- Picking Hoffman allows you to carry more molotovs, in case you need more chances.