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. So some quick tips for a nightmare run: 1. You actually have to do this anyway to be able to dmg the demon. Also make sure you are disrupting the rift whenever you can. If they are spread out and one of them goes down this will give you a better chance to run over and revive them.
Otherwise they will just stand right on top of the demon and get themselves killed. 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. By that time I had finally figured out the game mechanics and beating the first prologue boss was a piece of cake.
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. I don't think there really is a guide that can be given on how to complete this game on Nightmare mode.
