Update 28SEP2021:
It appears the game was updated and it will auto pop the achievement in the main menu if you had completed it but had glitched progress for this run, according to a couple of users who still had problems.
This achievement is for the base game missions only and can be done on Easy difficulty as long as Nightmare mode is selected. Also, much like the difficulty achievement for the campaign, I can confirm this can be unlocked by completing the last chapter of each mission as I did it like that solo. Nikoyroslav and another player mentioned they have gotten this coop doing this method, but it may be finicky. I know the difficulty achievement (for hard) had some issues with the achievement popping in coop when only doing the last chapter and the same may be true here since someone in the comments mentions it worked for them while another said it didn’t.
This achievement is bugged for many people and Gonzo points out in the comments that he managed to get it after deleting his save and redoing.
As far as strategy goes, recommend the following, in general:
-level up and get perks ranked up before attempting this as there are a TON of enemies and heavies. Specifics are up to you but for perks I used pouches, weapon focus expert, enhanced vigor, and second chance (all gold).
-learning to train is your friend. Train the enemies, take your time on objectives, and focus first on any ranged enemies. Gunners and flamers will damage you while training, as will snipers, so try to eliminate those first while training.
-Use checkpoints wisely. If you lose your downs from second chance and are in bad shape, reload checkpoint so each checkpoint you still have some resources to get you to the end.
- I used takedown frequently for armored enemies and to heal often. I also used divine blast for melee so I can could stun and heal as well. Make sure you are using these frequently as soon as the cool down is up so that you can stay healthy.
-Since the last mission is only 1 chapter, this mission can be done coop with no issues.