Doom 3

Nightmare is... interesting. Whereas the other difficulties just increase the damage you take, Nightmare has some special rules.

1. Your health, when above 25, will gradually lower until it is 25.
2. All health pickups in the game are removed, however health stations are still available.
3. You have the soul cube from the start of the game, allowing an instant kill of all normal enemies and a health boost (up to 100 from most enemies, less for smaller enemies such as spiders) every 5 regular kills.

25 health plus the huge damage that enemies do is enough for most enemies to kill you in one hit. As such you need to play very cautiously, use the soul cube regularly, and save all the time.

Below are some general tips I found useful during my playthrough. You will also want to check the very important warning in the level notes below for a potentially run ruining glitch that can occur in the Delta Labs.

General Tips:
Grenades: I didn't use a single grenade during my first playthrough, but they are invaluable in Nightmare, as they can be bounced around corners to damage enemies while they can't damage you. This is particularly useful for enemies such as Revenants and Mancubuses that have hard to dodge attacks but are relatively slow.

Corners: As most enemies can kill you in a single hit if you're at 25 health, your best bet for a lot of enemies is to run around a corner and then get a shot off on them as they round it. Grenades will kill most regular enemies in a single hit, or a well timed shotgun blast can finish off imps and regular soliders. For larger enemies, you probably want to get some damage in and then retreat to another corner. Just be careful when backing away from things as it's very easy to get caught on walls and end up a sitting duck.

Keep Backup Saves: Because you'll be saving regularly and are so vulnerable, it's easy to accidentally save right before a game ending hit. I used a rotation of 4 saves to ensure I could always go a step backwards if I needed to, but pick whatever amount is comfortable for you.

Cutscenes: These are unskippable and usually play before something big is about to kill you, so save as soon as you see the camera zoom in to the back of your head to avoid having to watch them over and over again.

Jump Puzzles: There are only a few of these, but the physics in Doom 3 are pretty atrocious. Save before and after these to avoid having to repeat them.

Level Tips:

Mars City Levels
- Possibly the hardest levels on nightmare, as you don't have good weapons and are up against hitscan (pistol, shotgun, machine gun) soldiers that can hit you instantly without a projectile to dodge. Save often, after every enemy if needed.

Administration Complex
- The first Pinky Demon (introduced in a cutscene) can kill you very quickly as it attacks you while you are trapped in a very small room. Hide in the corner next to the broken door, wait for him to leap through the window, and then quickly run out into the larger room.

Alpha Labs Section 4:
- Ignore the health station at the start of the level for now. Go free the man in the chamber, open the security locker in the room behind him for gear, then return to the health station and load up before continuing with the rest of the level
- Activate the EFR system instead of extending the service bridge when prompted, the latter spawns a lot of bonus enemies.
- Spider Boss is annoying, as most of it's psychic attacks have no warning and can kill you in one hit at 25 health. If you see an object raise into the air you can duck it fairly reliably, but make sure you stand up again quickly or the next attack will get you. You can try saving after every successfully dodged attack but be careful not to lock yourself into a hit. Make sure you use the health station right outside before going in.

Enpro Plant
- There are three plasma rifles near the start of this level. Picking up either of first two (on the ground in doorways) will spawn in about 10 enemies, be ready. Alternatively you can skip these and wait until Locker 63 (code 972) just after the first Lost Soul cutscene.

Communications Transfer
- When using the monorail/goods lift on the track, you'll spawn less enemies if you raise the lift just after the soldier appears in a closet on your left, go unlock the exit, then return and travel directly to station 3. Going to station 2 is just an alternate route to the unlock terminal but will spawn in some cacodemons if you select it.

Recycling Sector
- It's pretty easy to get lost in the many similar looking coridors. Try to avoid backtracking as it'll waste health and spawn in more enemies

- When you go down the lift and are attacked by two cherubs, your first mancubus fight is coming up. Sadly there's no real opportunity to heal up so you'll likely be going into the fight on 25 health or close to, which means one hit kills. Focus on bouncing grenades around corners and you can kill them without taking any damage. Take down 3 mancubuses and about 10 cherubs will teleport in. if you run up the ramp and wait they'll form an orderly queue for you.

- After the above fight, some more cherubs, and reaching an elevator, max your health out at the station to your left before using the lift. At the top is a damage over time area, where you have to use 3 computers quickly before your health drops to 0. Right after that you get to face two revenants back to back, just for fun. Roll grenades around the corner as your health is likely below 25 by this point.

Monorail
- Don't bother with the health station before boarding the train, you'll be back to 25 health long before your next combat encounter. Save as you exit the train to avoid having to sit through it again when the surprise supersoldier on the other side of the door kills you. Get a corner between you and this guy as soon as possible.

- You'll likely earn a soul cube while fighting the many spiders after opening the airlocks for the monorail. Save it, it's only spiders until you're back on the train. Right after you exit the train crash you'll have a supersoldier charging at you, soul cube him for an easy heal.

- Sprint past the automated turrets into the nearby office while they're busy killing enemies, and then just let them do their work before disabling them.

Delta Labs 1
- SOFTLOCK WARNING: This is HUGELY important, as missing it caused me to have to repeat 2/3rds of my entire Nightmare run.

This is an old bug that dates back to the original launch of Doom 3, but I sadly wasn't aware of it. When you first enter Delta Labs the power will be off. After crawling through a vent, you will reach an elevator.

DO NOT INTERACT WITH THIS ELEVATOR UNTIL AFTER YOU HAVE RESTORED FULL POWER.

Using the elevator control before you've been to the reactor core will cause a bug that means you can never use the elevator, which is requierd to progress further into the level. There is no checkpoint save far enough back to save you, and unless you keep a save from at the start of the level, you'll be out of luck and have to start your entire run again.

- Heaps of Super Soldiers in this lever, save frequently and have a rocket launcher or your soul cube ready when opening doors.

Delta Labs 2a

- Shortly after speaking to the scientist in the teleporter room for the first time, you'll jump over some crates and come face to face with a Chaingun soldier, a very tough fight at close range on Nightmare. He spawns when you jump on the crate, so immediately back up and throw grenades over the crate at him. After you kill him, a revenant spawns in, the same strategy works.

- Two imps spawn in behind you when you send the crane to location 6, although it takes longer it's better to wait and take them out and then redo the puzzle rather than try to fight them from on top of the moving crane.

- BFG room code 931

Delta Labs 3

- You're going to lose all your ammo at the end of the next level, so may as well bring out the big guns if you've been saving them.

- The teleporters are annoying, unless you're hunting for collectibles you may as well go directly to the three required locations (pad 2, pad 3, pad 1) otherwise you'll be forced to go through the entire level up to 3-5 times to see every room.

Hell

- This is a long slog, but all your existing strategies work here. Grenades around corners, use your rockets on the mancubuses and barons of hell. When you get to the circular room with a BFG cell sitting near a cliff, hug the edge and look to the right for the BFG, this makes the upcoming boss fight massively easier.

- Do a charged BFG shot on the exposed energy core as soon as the fight starts, then run around the corner to your right to hide. Make a save here if it's clear. If you're lucky all the seekers will come around the corner and be bunched up. Reload your BFG and take them down with one shot, then do a follow up charged BFG shot on the core to take the guardian out straight away. You don't want to fight this guy on his terms on nightmare if you can help it.

Central Processing:
- The Sarge/Saboath fight will be over in 5 seconds, with either one of you dead. Make sure you have a soul cube going into this fight or it's over before it starts. Once the door opens throw your soul cube, and then immediately fire 5 rockets directly into the boss. The soul cube will charge your health, the rockets will (hopefully) negate his first BFG blast, and the rest of the rockets will finish him off.

Site 3
- Another frustrating potentially run-ending glitch, however this one unfortunately can't be prepared against. There is a ladder into a duct (just before you meet Pierce Rogers) that you will sometimes get stuck on, preventing you from climbing into the duct. This took me about 45 minutes of reloading, trying to toss grenades below me to boost me a little bit higher. Make sure you have BFG ammo for the fight after this, as you'll almost certainly be low health if you had to grenade boost up the ladder.

Caverns 1
- Many many spiders when you exit the elevator. Honestly it's safest just to go right back into the lift since they can only come from one direction then.

- At the bottom of the lift you'll face a minigun soldier and you'll likely be at 25 health due to the long ride. You can use a soul cube on him if you have one, but it might be worth saving it for the Arch-vile that teleports in once you jump down. If not, hit the vile with 3 rockets quick as you can, then run under the walkways for some cover against the Hell Knight and the Imps.

- Try and save a soul cube at the end of this level. You'll need it for the start of the next one.

Caverns 2
- Pretty constant fighting but nothing too complicated if you've been through everything else. Use as much ammo as you want, you'll get to refill before the final boss.
- There's another spider boss in this level, but at this point you can just hurl bfg shots at it.
- Ideally go into the final boss with a soul cube saved, as this will make the fight much shorter.

Cyberdemon
- Comparatively easy after all you went through. Keep him on the other side of the stage, fire your BFG or rockets at enemies until your soul cube is ready, then fire it at the cyberdemon. Repeat until dead. Watch the unskippable cutscenes, and achievement unlocked.