This game is hard as hell and the drop rates for Red on master are so unbelievably low!
In nightmare, all the enemies have ice on them that must be damaged first to get any head shots or do damage (Relatively Speaking). In addition, the attacks they inflict do a ton of damage and you can get smashed into the ground in just 2-3 hits! Worst of all, if you all die, you lose your weapons and you MUST re-buy them again from The Line!

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The best way to kill enemies in nightmare is to have the prototype weapon upgrades or Red Floppy disks such as the barrels and batteries.


Batteries
These allow for critical damage (10% Chance) against Snowbreed and can be very useful when clearing out big enemies like the Jugs, Brutes, and Eaters.

Barrels
These give your gun extra damage per shot which is useful for taking out large groups of enemies!

Ammo Counter
GET RID OF THIS ATTACHMENT!!!! This reduces Stopping power on your guns and makes them weaker. Just listen for the click, then reload. Simple as that.

Holster for guns
I usually do not use this as it is too slow for me to change out weapons and is a real headache. I use the weapon wheel option as it is much faster to obtain your weapon. However, this is ultimately your choice and either will work for nightmare runs

Reload Style
USE AUTOMATIC RELOAD! Believe me, a few bits of harvest are not worth the manual reload. You'll get a lot of harvest just completing a nightmare run so don't bother with slow reload. You will need to be fast in nightmare mode to survive so just use automatic reload. be the badass Rambo!


First and foremost, you need to play with at least ONE other player to do this run
In order to "cheese" the game you need to learn how to door glitch. THIS IS ESSENTIAL to an easy nightmare run. One player will always stay inside the safe room WITH THE DOOR CLOSED! Even if the player(s) outside the room die, the enemies will all die within minutes! Yes, the enemies die when they cannot reach the other player who is inside the safe room (at least for now until a patch)!

With that said, the other player(s) who are not staying in the room must door glitch outside to start the run. You will die a lot if you do not have the ideal setup (with reds) as stated above but it will not matter if you are doing "easy nightmare" to farm them.

You will continue to go through the level until you reach the next safe room! Once you have cleared the enemies (or they die) and open the next saferoom, the player from the prior saferoom is safe to open the door and make their way to the that saferoom. You will repeat this until you reach the end boss (Does not apply to the final mission) area and clip into the door to end the level.

And that's all you have to do for an easy nightmare run!

Grind before any patch! I await to see how badly this guide ages.