UPDATE *These videos were made pre-September update where enemy speeds were bumped up*
Here is a three part video series I made of myself doing Nightmare co-op solo. A lot of practice is going to be required to get though E1M3-E1M7 as this is the hardest section of the game to do on Nightmare. If you can get through these levels everything after is super easy.
General Tips for the Episode:
1) There are a lot of enemies in some levels, this is not as big of a problem as you might think. more monsters = more infighting. In most levels this provides a source of shotguns without you having to do anything.
2) There is only one speed running technique required to finish the episode. It is found on E1M4. You can practice it on single player using level select and the lowest difficulty setting.
3) Turn up the level and screen brightness to 100, this turns low lit rooms into fully illuminated ones. However, there are still going to be rooms that are relatively dark still.
4) The two biggest threats in this episode are Pinkies and explosive barrels. Both are annoying body blocking objects only meant to get in your way.
5) The shotgun is your best friend for most of the levels here, unfortunately it is extremely inconsistent with the damage it does to enemies. Sometimes you will 1-shot monsters, others it will be two or three times you need to hit them.
6) The longer you take on most levels the less likely you are to finish them.
E1M1 (00:01): A pretty straight forward level, run from start to finish. There is not a whole lot to the level.
E1M2 (00:59): Another straight forward brute force kind of level, this should not be too much of a problem here.
E1M3 (02:36): The levels start getting tough here. This is a heavily brute force reliant level, nothing special other than you will most likely die a few times here before getting to the end.
E1M4 (04:10): An easy level if you can learn the jump to skip the majority of the level. There is an in video explanation of the trick, otherwise brute forcing this level is a nightmare.
E1M5 (06:08): This level took the longest to figure out a solid route for. There are a lot of enemies and you can die very quickly. Follow how and what I use for cover, make sure to clear out the hallway after pressing the red switch every time you end up back there after dying. Time is a huge factor here, dying too much will make the level essentially impossible to finish. Do not hit the switch in the room with the blue key, it will ruin your run. The ending is also a pain, the room is dark even with maxed out brightness settings. Go around the left side as it is quicker to get to the exit. You will be blocked by pinkies and their invisible counter parts as well as barrels in this level.
E1M6 (09:58): The start of the level is pretty easy but quickly devolves into high skill and luck. Any deaths after picking up the first invisibility item will require you to restart the level as it is insanely hard to make it through afterwards. This level will take some practice to get through.
E1M7 (13:38): This level combines brute force and careful planning to finish it. The first bit with the yellow key can be brute forced, just keep trying until you get the key. The second portion requires knowledge of the enemy placement coming up, watch where enemies are and how I dealt with them. You should have the red key now. Brute force into the blue key, this is easy as it next to the spawn location. You will need to take the secret to grab an invisibility power after flipping the switch to raise the door that leads to the exit as there are a ton of hit-scan enemies here.
E1M8 (20:25): Probably the easiest level as it is essentially a boss fight you can keep throwing yourself. There are respawning shotguns and chainguns before the elevator up to the bosses. If you die you will be sent back to the platform at the start, shoot the pinkies in the pit until an opening appears and make your way back to the weapon rooms and then to the bosses.
Every level in episode 2 and 3 are a breeze compared to before. Follow the routes I take in these levels and you should have no problems finishing them. I managed to beat a lot of these levels first try or with minimal effort on my part.