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This is a bad boss. Not because it’s too hard, not because it’s so easy with the right build, but because it’s not made for multiplayer... in a multiplayer focused game. The nightmare gains way too much health with 3 players. The void mechanic Makes it so only one player gets to fight the boss at a time and nothing make sense the first time it happened. The group I was with spent 30 mins on the first attempt against the nightmare( this is the same 2 guys I played through the whole game with). Whenever someone got voided they ran for the exit because we thought it was an attack to separate the party. We saw we were doing damage even if the bar wasn’t going down so we figured we needed to stagger the dreamer out or something. I only learned how the fight was meant to work after looking here. I still like the game but jeez this is a very poorly made fine boss. It doesn’t at all consider its in a co-op game
So if you are looking to be done with this boss quickly, I used the crossbow to kill adds in the void (one shot them) I had swarm and breath of the desert. After killing sufficient adds, I usually got 8-10, throw swarm on him, the swarm keeps Breath up at 3 stacks pretty much the entire time, and opens his weak spot each time. Solo, I killed him with 1 void visit. with 3 people I was getting him down to 1/4 to 1/3. I did not shoot my guns at the boss. Just use swarm, then breath until his damage buff wears off.
For anyone struggling with this boss, take a one shot weapon (sniper, crossbow, hunting pistol) and the fireball mod from the 2 fire bosses 1. Phase: just spam headshot with the weapon, ignore adds 2. Phase: wait till you get into the other dimension, kill some enemies with the one-shot-pot and exit, then spam all fireball stacks (the fireball stacks will recharge themselves after a explosion, so just spam them till the boss goes invincible again, fireball is legit broken here)
used swarm to spam black holes that were doing 99,999 damage
Very easy in solo after you understand the mechanics of battle.