Hey fellow hunters, in order to obtain this achievement you need to go the Zombie DLC at Jakobs Cove. Once there, proceed to Hallow's End. (My Current Position on the map)
Once in Hallow's End proceed to the beacon I have placed down and DO NOT PICK UP any brains until after the mission starts this will save time as they do not de-spawn.
Once you are in the area you will see a Fyrestone Sign on a building, head towards this building. The questline will start right after a short cinematic.
The quest itself has multiple stages in which you will have to collect brains.
The number collected goes from 10 -> 25 -> 50 -> 100 -> 250. You will have to collect a total of 435 brains so I strongly urge to do side missions whilst doing this in the background.
For stage 1 and 2, I would recommend finishing those why you are there.
For 50 Brains, I would recommend going back to Jacobs Cove and cleaning the area while doing other side missions.
For 100 Brains I recommend going to Generally Hospital then back to Hallow's End.
For 250 Brains, I strongly urge going to the lumber yard as you can normally get 100+ brains here. Once you get to the part where it turns into another enemy type, either exit the game and repeat the grind in lumber yard then heading towards Hallow's End or turning around and heading to another area.