This guide is split between 2 parts, if you're if you're looking for an achievement not featured in this part, view the other part:
"Energized" will be for using 100 batteries. I didn't use nearly this many on my first playthrough, but while going through my nightmare run, I finished it up. Just remember they still count between playthroughs, so you don't have to actually use 100 in one playthrough.
Then "Child of the Elder God" is for having a rock n roll moment without dropping to a low health state. This isn't too hard to do even on nightmare mode so as long as you're somewhat careful this should be pretty easy to do. Just make it through the sort of boss battle on the stage.
"An Idyllic Small Town" is for making it through "Night Life in Bright Falls" in one go without dying or restarting even once. That means starting at Episode 5 in the first chapter "Night Life in Bright Falls" you'll need to make sure you do not die and do not restart a checkpoint. This isn't too bad on easy difficulties, but the only places you might run into issues are during the church defense which won't be too hard as you'll have Sarah helping out. Just make sure you have some grenades to use and ammo.
Then at the helicopter defense again just make sure you have grenades and flares ready. Turn on both of the flood lights and keep alternating between them to keep them up. Then shoot the taken you can and use grenades when needed. Other than that, it shouldn't be too hard.
For "Gunless Wonder" you'll need to make it to cauldron lake without firing a single shot. This will be in the first part of Episode 6. You'll be driving your way to Cauldron Lake and will have to either flare, flashbang, or use the car all the way there. It really isn't that bad for this one either since you have cars to use most of the way.
"Meet the Deadline" will be for making it from the coal mine museum to cauldron lake in 30 minutes. This will be one you may want to ignore and then reload the chapter to do after the fact so you can solely focus on it. This will be in Episode 3, part 2 "Mirror Peak" and you'll want to go as fast as you can to get to the end. Eventually you'll have the achievement to pop up.
Now we can get to our second playthrough - the Nightmare run. Of course, we can't start this until after we've already beaten the game once.
There are a few general guidelines to go by to get through this. It's not super hard but can have some difficult parts if you're not preparing. You'll take more damage from hits, do less damage, and there will be less supplies.
That means you'll need to try to conserve ammo and if you're going to use it you better be hitting your shots.
Generally, you'll be better off conserving stamina when walking, and once enemies start coming then sprint and dodge and run towards the light. If you need to then use your flashlight to keep the taken off of you or use a flare to make it to the next safe haven light. You don't want to take fights that you don't have to.
You also don't want to be going out of your way to pick things up that you don't have to other than the 14 nightmare mode manuscripts that we'll need to finish up the collectibles. Like I mentioned before, I'll have a full video going over all of the collectible locations in depth so be sure to check that out while doing these runs.
Then once you beat the game on Nightmare mode, you'll get all of the difficulty related achievements. That'll be "Departure" for beating the game on Normal mode, "Hardboiled Writer" for beating the game on hard difficulty, and "Alan, Wake Up" for completing the game on Nightmare mode.
And there you go! 100% of the achievements in Alan Wake's base game.
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