I am reposting my guide from the Xbox One version to hopefully help people with tips on how to successfully complete this game on Nightmare mode!
I don't think there really is a guide that can be given on how to complete this game on Nightmare mode. I mean there is no do this ... and you will get the achievement. So I am going to give my top tips on how to go about completing this crazy long game on Nightmare:
I've played both of the other Dragon Age games as a dual wield rogue, but this time I could not get past the prologue boss on nightmare with that setup so I rerolled and went with an archer. By that time I had finally figured out the game mechanics and beating the first prologue boss was a piece of cake. Have Cassandra tank the pride demon and make sure Varric and Solas are positioned at a good distance from the demon and make them hold their position. Otherwise they will just stand right on top of the demon and get themselves killed. If they are spread out and one of them goes down this will give you a better chance to run over and revive them. Also make sure you are disrupting the rift whenever you can. You actually have to do this anyway to be able to dmg the demon.
So some quick tips for a nightmare run:
1. I really wanted to play with some of the other characters but I found it just easier to play with Cassandra as a sword/shield tank, Dorian and Vivienne or Solas as the other party members. I went with an Archer/Assassin. Stealthing as an assassin and then using Overdraw is just a beast.
2. In some tough areas, I'm mainly talking about main story missions cause you are locked into them once you start them, make sure you are fully stocked before you start the mission at the war table. I'm talking about grenades and rejuvenation potions. If you do not stock up on these before story missions you are SOL and will not be able to have any on you!
3. In story missions make sure to heal up (if you have any potions left) before you click on the resupply crates. Also, you should pick increasing the amount of health potions you can carry as soon as you can as an inquisition perk.
4. In some areas that were difficult (and seeing as I was playing as a ranged rogue character) if your characters are starting to go down, you can have your character stealth and run away into an area you have already cleared. If you run far enough your 3 other party members will revive back by you and even if their health is low you can send them back in to damage the enemies. The enemies do not regenerate health. I think this is my biggest tip. This saved me on numerous occasions. Especially at the beginning of the game in the haven mission. You have to keep trying to load this trebuchet, but everytime you do more enemies swarm you. You can run up a hill next to you and take out the enemies much slower than just getting spanked by them. Do this and it will make the nightmare run much easier!!
5. Make sure you are the right level for the mission you are about to start. At the war table it will give you the suggested level for starting that mission. If you're playing on Nightmare make sure you are at the higher end of that range. If the mission is recommended for a lvl 8-10 then go do side mission and stuff until you are closer to 10. Heck you can even over level and make the story mission even easier if you want. If you are going for the full completion then there is a ton of side stuff you need to complete anyway, so might as well get to it.
6. This achievement is for finishing the "main" campaign on Nightmare mode. This does not mean that you have to get the achievement for killing all 10 high dragons etc. while still playing on nightmare mode. When the final story mission is over you will end up back in Skyhold and can continue on with any side mission that you please. As soon as you kill the big baddie at the end the achievements for complete Nightmare and Hard will both pop assuming you never ever changed the difficulty level. The only side missions that you get blocked from are romance related ones and companion related ones. So by all means complete the main mission and get back to Skyhold. After the credits, make a save. Then switch to Casual if you like and go to town on those dragons or whatever else you have left to do.
7. I'm putting this one in here just in case it has a huge impact, cause for my story I think it did and it ended up being a side quest but it had an effect on a battle, so I'm putting it behind a spoiler for you just in case you don't want a story spoiler:
8. My last tip: If you make the right decisions in some story missions you can skip the boss fight completely
Have fun on your Nightmare run, I know I did