Any tips for the Nightmare Co-op Trophy? I don't have much of a prayer left for that one.
I did it solo.
Take advantage of the fact that you infinitely respawn.
Punch holes in enemy formations, and in most levels you can make it back to where you died before they respawn and go farther.
Infinite enemies means infinite ammo.
Watch Nightmare runs on youtube to get an idea of a good route.
Almost all levels in Doom 1 (haven't done the Doom II trophy yet) can be brute forced like this.
Just get that key, and it doesn't matter if you die in most levels, as it'll just usually bring you closer to the door if anything.
There is a level or two where it's vital to grab an invulnerability power up and get to the exit on the first go. For these, you'll want to restart if you fail, since Brute forcing won't be possible for most people.
Most of the levels aren't too difficult. I really recommend playing through the game normally immediately before going for this so that all the level layouts are fresh in you mind.
Knee Deep in the Dead is actually the hardest episode for this because of the closed spaces.
If you actually have a second person to play with you can just both brute force and it's fairly easy.