Early on at least I don't see how Nightmare is even possible without generous use of the tactical camera
Knowledge and kill order, pretty much. If you know which target are problematic, you can take them out rather quick. The first time I went through Nightmare I had big issues early game. Now that I've gone through the game a few times, and done 1-11 100% pre-skyhold about 9 times, I got no issue with it. The only issue I ever had was on my not-lucky DW rogue in a no-crafting run. At level 6 I went to the western are of Hinterlands, where stuff are level 8. There is a quest to kill 3 Greater Bears. To my fear, all 3 were on me at once. I had level 3 daggers and did between 1-3 damage per hit. Varric did 23 damage per hit. I had the wrong staff on Solas, and no offensive abilities. He did 1 damage per hit. That **** was... not fun.
Other than that it's smooth sailing early game.
Archers > Assassins > Mages > Warriors > Maulers and Terror Demons > Wraiths > Despair Demon > Rage Demon > Greater Demon > Lesser Demon. That gets you through 1-13, which is the whole early mage. Then you get specialization and you just won the game.
Note, this is without extensive use of tactical camera. I only use tact cam a few times during the game: 1) When Cassandra is tanking like an idiot and got her back towards half the enemies 2) when ranged go into melee, when I want to pigeonhole enemies into a corner(where my tank is blocking) and 3) when I fight dragons. (Or dragon. Only Ferelden Frostback can be done early-game, and is the only one even remotely difficult!)