Fallout 76

This achievement require you to use fire energy weapons on overgrown mobs.
Overgrown mobs are found all across all 3 Atlantic City areas (including during the questline from manor ingram) , but here some global location indications :

• Can be find at the start of the The Most Sensational Game (in the first part of the entrance area, around 10-20 overgrown, can be more with the overgrown objective at the start)

• Can be find also at the start the Tax Evasion (more on side parts of the entrance area and upstairs (around 5 - 15) )

• The best is for sure during The Human Condition (close to a solid 40 overgrown if alone to farm them for the achievo and if doing the whole expedition)

About fire weapons, for some security in my opinion, i ve used a standard / regular flamer. (Edit : Thanks to fabian907, as he confirm me the Holy Fire also count for the achievement.).

Hope it help you out, let me know if they any details i miss to say, thanks for reading.

Tip from Boss Complex (thanks to him to allow me this edit) : "i didn't have a good fire weapon. there are 2 Flamers in the basement of the Neapolitan Casino. i grabbed the one in the laundry room. easy enough to find."

PS : Plasma flamer never counted as weapon with fire damage properly said or the perk called "Friendly fire" would works with those weapons.
Also, the very last thing i did miss to say in case you dont know it, but if you are in the case of farming the overgrown on the same expedition, you can leave and restart a new expedition as the achievement keep the progression of the kill track independtely of where you kill the overgrown.
I got confirmation, the melees who use additional fire damage arent counting for this achievement sadly.

30/03/2024 Edit : the cremator (new season atomic flamer gun) doesn't count for the achievement for some odd reasons. I had some closer friends who have tried to do the achievement with it and didn't make it.