Don't even need a healer for nightmare, insta-focus elite rogues+blood mages/qunari mages down (petrify + unleash x-class combos on them). Pull small groups out of packs by making your guys hang back and run JUST into view then back.
Rock Wraith wasn't hard as such... just a PAIN for positioning.
Yeah the hardest thing in the game is "trash" groups with a blood/qunari mage or a few elite rogues, if you can focus them down though it's not too bad. (although last boss somehow 1-shot aveline... was all O.o)
Aveline - Bodyguard on fenris, 2 tanks. Rally + Turn the Blade/Elemental Def for surviveability.
Fenris - No aoe abilities except spirit pulse, which you have to be fucking careful with. Rally + Might for imbaburst.
Varric - Pewpewkachoo knockback shot is sex.
Force/Spirit healer (for knockback resist, more cc & mana/hp regen bonus, never actually got spirit healer healing things) me. Primal specced for no FF aoe, stonefist, rock armor etc. Heroic Aura, Heal (for a quick heal if really needed), Party Arcane Shield. And Winter's Grasp/Cone of Cold are amazing
Yeah you go through alot of potions, but the less you have it seems the game makes them more likely to drop. Also remember revive grenades
And don't cheap-out on constitution, it's your friend.