I run a 2 warrior 1 mage and 1 ranged rogue setup on nightmare. Granted my mage is a knight enchanter so im pretty much adding an extra tank at that point... anyway, i run cassandra sword and board tank, iron bull as my two handed warrior (more on him later), Sera or Varric as my ranged rogues, and my inquisitor as a knight enchanter mage.
I start off by alternating between vanguard and sword and shield trees for cassandra, then after ive gotten all the passives at least, i begin to spec into templar. I have charging bull disabled, but still on her ability map so i can use it manually to build guard in a pinch. I have challenge and war cry preferred as well as shield wall. She has a dispel ability in templar spec, so i dont have to bring another mage with me to use it for rifts. I outfit her with the best armor, a good shield, and a strong weapon that usually adds to stamina or something similar. I dont do guard on hit with her because its useless as she generates so much guard anyway.
2 handers are ones you have to make sacrifices on in nightmare... I always start iron bull in the sword and shield area first, yes you heard me... i get him turn the bolt and bears maul the wolves, if only for the ranged defense increase and flank immunity which is entirely necessary on nightmare. Then i go into 2 hand and battle master. I alternate here and usually go towards war horn and its upgrade for the guard damage. much later around level 15 or so i start hitting the reaver spec. I outfit bull with the best armor and a heavy hitting AOE 2 hand weapon both with guard on hit, i do manage to get him war cry so i can draw attention from my weak party members in a pinch and a half assed guard generator, but i dont go further than that. I usually set his potion use down real low, like dont use it until 10% health, as his reaver spec rocks at low health. I have war cry on the ability list, but i have it disabled so he wont use it unless i tell him to.
I tend to use sera later due to me hating the artificer class. I usually fill out the ranged tree first, but my first points are always stealth and poisoned weapons. I have stealth and poisoned weapons preferred. Full draw with its upgrade is nice, as it puts the enemy to sleep and coupled with my KE mana blade creates the nightmare combo for great damage. When i get to skyhold, i get down to flask of lightning as quick as possible. now i know some people say aw that flask sucks cause it slows down everyone except sera and its just giving her her normal damage output. You're wrong, dont play as her and put it on preferred and you will see what i mean, the world will not slow down for the other characters like it does when you play as her, alls you will see is about 15 arrows fly by in rapid succession and you think wtf is that... thats lightning flask.
Mage.... I spec far into spirit... i usually avoid revival though, as i never really use it. Keep dispel on map at all times, this is great for rifts and mage barriers. Then i usually put at least one spell in each elemental category for diversity till i get my specialization. Once i hit knight enchanter, i get the blade and the left side of the tree so i can generate a barrier with each hit, then focus on the other side. I usually disable the blade when im not controlling my mage, as they tend to get themselves into trouble. Anyway, cast barrier on yourself and maybe armor with a small guard on hit bonus and watch yourself tank a dragon on nightmare with no party, KE is seriously OP.