I need more tips on Nightmare, haven't started yet but I don't really want to...however as a trophyhunter I can't let this go since it's the only two trophies left for Dragon Age Inquisition. I do hate it to be honest, especially since the first and second games did not have trophies/achievements regarding difficulty. And yes I know DLC had them, but that's not the same thing. I'd rather walk 10 hours in the Hissing Wastes listening to Vivienne and her "My Dear responses" while Sera is making pranks and Cole is losing his mind than playing this game on Hard or Nightmare.
I have been searching a lot for this, it seems you need at least one mage in your party at all time and someone who can tank. However that leaves two other people. Most people advise a second mage or a second tank, I hear that the skills "Barrier, Dispel and Revive" are a must have. Lastly if you only play main story missions and don't do anything else it will go quickly, but is that possible since you miss a lot of items and XP that way. I want to enjoy this game a second time for again 100 hours and more but first I need to go through it fast on Nightmare.
I feel already unwell thinking about it, make's me remember Call of Duty World At War again on Veteran, now that was a nightmare. I don't understand people want to be irritated or even annoyed while playing a game, because that's what you get on these difficulties. I know it's a challenge, but it's a challenge I'd rather avoid.
If you merely want the achievement, there are a ton of posts about Exploits/Glitches/Etc. that will get you just that and though I don't condone it and in my opinion invalidates the point of the achievement, it will get you there if that little ding sound is your goal. HOWEVER, if you want to actually play the game with a challenge, the only really hard part of Nightmare as many people have pointed out is pre-skyhold because you don't have a lot of points, and need to tactical spam as much as possible.
Most of the time I was able to not have to micromanage my companions, but you do need to tweek their AI a bit in terms of the HP threshold they use to chug a potion or if you want to manually control it, and always manually use dispel and barrier. Two mages pre-skyhold and Cassandra tanking is all I needed and everything else was cake and golden. Honestly it's a joke with three mages since you cycle through 3 barriers, and the only hard spots, in my opinion are large groups which can be cc'd with ice, and dispel for dealing with rifts.
Focus fire is key and was more effective in my experience than using AOE, but that's just me and my playstyle.
Here is an example of the Fereldan Frostback Dragon Fight on Nightmare w/ an AT LEVEL team composition. Knight Enchanter is strong, and stupidly trivializes things in the end of the game to get a feel for what's in store.
Good luck 
https://www.youtube....h?v=-V9wc6C-AGU
(All melee party, Nightmare Frostback Fight, No Exploits, No Dragon Runes, with a Fire Staff -1 damage basic auto-attacks)
The point of this video is that it's not so bad, and party micromanagement doesn't have to be a thing if you don't want it to. You don't need to out level content, nor stack your gear to each boss fight. All you really need is to know the fight and how to adapt with what you have.
hmm currently lvl 8 guess i should do another map before trying again, what is better for leveling up, clearing the storm cost or the gloomly swamp of undead?
I just did Hinterlands quests as my Nightmare playthrough is my completionist playthrough. I did the game on Normal as a warm-up so I knew where potion bottles were ahead of time and got a general sense of when a checkpoint was coming up so I knew when to save them vs. chug them like no tomorrow.