Dont use cheap tricks like assassins mark or thousand cuts. Thats more a recommendation from a personal opinion that they make the game too easy. Anyway here goes:
1) Take your time early on. Rifts are permanently shut down, so no need to rush. There is also no shame in swapping zones back and forth.
2) Hardest part imo is pre-skyhold. Party:
A pure fire mage, fire mine and fire wall are extremely efficient spells. Another solid combo is static cage + energy barrage.
A SnS tank
A support mage, I recommend Solas as a barrier + ice mage for winter grasp. Grap fade leap as an escape for him.
+ Inquisitor. If rogue, go archer early then swap to DW at skyhold.
Set shield bash and payback strike to preferred.
Set static cage and fire wall to preferred.
Set stamina on tactics to 0%, you want your party to always spam abilities.
On solas you might want to set stamina to 10-20%, so you can dispel emerging enemies on rifts. You probably know this but you can dispel demons being summoned by rifts.
Later on you want Cassandra or Blackwall as your tank. Blackwall is tankier, while Cassandra provides party buffs to damage + resistance and has the all-mighty wrath of heaven + spellpurge
3) Loot all crafting nodes you see. You want to always have 5 x regen pots at hand and/or later on - Bee grenades to panic people. Upgrade potions, first priority regen pots. Resistance tonics are also king.
4) Explore everything and loot everything that you CAN at your current level. Nightmare is an enjoyable slow-burn imo.
5) DELAY THE ATTACK ON HAVEN AS LONG AS YOU CAN - It is the hardest point in the ENTIRE game. When you DO get there, take note of the supply caches, one by the alchemist. One by the blacksmith, one by the last trebuchet, one at the chantry entrance. DO NOT take them if you dont have too, save them. On the last boss in Haven, DISENGAGE and run BEHIND the chantry - it will pull Fiona/Samson with only 1-2 adds.
6) After skyhold, craft your guts out.
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I completed Nightmare without using tactical cam. The key to victory is mosty:
CC/Disables like Static cage, winter grasp, fire wall, shield slam, payback strike.
Dispels.
For an easier time run a party with:
2 x mages 1x warrior tank 1x rogue.
Min/max:
Varric | Cole | Sera
Vivienne | Solas | Dorian
Cassandra | Bull | Blackwall
Cole has a useless ultimate but great skills. Sera has a great ultimate but mediocre skills if not micromanaged.
Vivienne has a great ulti, Dorian and solas has mediocre ulties but Dorian wins out.
Cassandra and Blackwall are both great, most people would argue that Blackwall is the best. But Cassandra has another AOE dispel, stun, party buffs, and better ulti.
Edit: lastly , the Inquisitor.
Most powerful in skilled hands is the DW rogue.
Playing the tank as the Inquisitor is also a very fun and solid choice.
Mage is always bulletproof. I recommend Necromancer or Rift mage.
Some people will argue that KE is OP. Keep in mind KE is a dreadfully boring spec, and in the right hands - (well, my hands) a DW rogue is imo the strongest inquisitor class in the game. A DW rogue has much higher damage potential and great CC + escapes and is imo also the most fun class in the game.
"Knight enchanter OP" is a very exaggerated statement. When the KE becomes as powerful as it does, a DW rogue is in the right hands - just as powerful if not more. Hell any class at that point in the game is equally OP due to crafting. A crit build Static + EB build even WITHOUT a specialization will do more damage than most, but that is lategame only.