I originally got this achievement on Nightmare Caves after many tries with 72k thanks to RK1 Kl0ndyk3's solution, but I tried Nightmare Ghost Town afterwards and got 92k quite easily on the first try with some mistakes so I would recommend this.
Heres a few useful locations of items, based off of where you start out:
- There is a flare right in front of you on your left. After you get that, go into the alley beside it and you will find a sawn-off shotgun.
- The further building on the right is where the ammo box is. Beside it is the flare gun, which I would recommend you pick up next. There is an extra shot nearby. From here, cut through the alley of the ammo box building. You will see a set of stairs. This is where you must hold out, but not yet.
- The far right corner of the map from where you started holds the hunting rifle. Right behind it is a couple of flares. There is also hunting rifle ammo nearby.
Now you may have to look around a bit more but it is pretty easy to have 5 flares by now and 2 or 3 shots for your flare gun. Go up the set of stairs on the side of the ammo box building, and use the sawn-off shotgun. It is good against pretty much any enemy, but you must keep close to them. When you run out of ammo for it, you can use your hunting rifle shots. Once you are out of ammo completely, then you must go to the ammo box. Use a flare to get off of the staircase and another flare when you are near the ammo box to grab the ammo. You will probably want to use another to get back to the top of the staircase. But then you should be able to hold it for quite a while. When the huge guys with saws come around, you will want to use your flashlight to remove the darkness, then shoot them with the flaregun to not only cause significant damage, but give you time to kill them. Hopefully you have a nice multiplier at the end that would let you get a big boost in case but as I said, I got 92k first try (~66k before end) so you probably won't need it.
This does leave you with a couple flares if you need them to fill up your hunting rifle or hold your multiplier at the end. You can also use this to hold them off while you look for a grenadier who is generally visible from up top.
Grenadiers can sadly still hit you but its tough for them. If you are at the top then it takes pretty much a perfect throw for them. If you can hold midway up the stairs then they tend to hit there and it bounces down to the bottom, so you can go to the top and he will kill his teammates for you (this does count to your score/multiplier)
Hopefully this helps. Good luck!