To get this achievement you need to have Expeditions unlocked by completing the "Responders Reborn" quest.
In order to get the achievement, you need to acquire a flamethrower. There are two possibilities: one, you buy a plan (which is the expensive method), or two, you go to "Quarry X13" in Cranberry Bog; behind a building, there is a free flamer you can pick up. All you have to do now is get a lot of "fuel."
Start the quest "The Human Condition" and go nuts on the "Overgrown." Be sure to kill the "Polinator" first because it acts as a healer for the "Overgrown." If you finish the quest and you didn't unlock the achievement, just "Fast Travel" back to "Appalachia," then go back to AC-Flooded City; the Overgrown keep respawning.