General Information for Nightmare as Marine/Prey
Welcome to hell private. This campaign is by far the hardest to beat on nightmare. Aliens are more durable as on Hard and the final mission is suicide. You have to fight: 30-50 Aliens, 2 Praetorians, 8 Androids with cloaking device and shotguns, which can kill you with a single hit if you are at least 5m away and the infamous new Bishop with a shotgun. He takes 16 Shotgun hits if you are up close on Hard, so I presume he takes as much on nightmare and finally the cutscene where you have to aim fast to kill him.
Some tips I can give for survival are: Choose your weapons wisely. Scoped Rifle can kill an alien with 1-3 well placed shots, shotgun can knock them back if they are too close. Flamethrower is working awkward and Pulse Rifle is to weak against several aliens at once. Another big problem which will be really frustrating: An unintended head shot to aliens if they are right in front of you is instant death.
Once you encounter Androids be careful. They kill you quite fast and take 5 shots from the Scoped Rifle. So try to destroy a leg with two hits to immobilize them. A headshot helps as it weakens their accuracy. If you stand still they can't target you with the Motion Tracker.
To kill the Praetorian in the Research Lab use the Frag Grenades from the Pulse Rifle and the alternate fire from the Shotgun to bring it down.
A good to perfect knowledge of the level is necessary as well as some real good skill.