Nightmare got much easier when I found a
thread on the forums that tabulated all the enemy resistances, and began to set up my characters for certain encounters with runes (armor and weapon both). Once I began to incorporate that, it became so much more manageable. I mean, my character is glitched right now (because of Sebastian) with -55% damage reduction, and I'm still able to get through the game on Nightmare.
This is, honestly, the first game I have
ever played where I didn't horde potions, where I actually have to both buy them and use them, and where I actually worry about stuff like runes.
Just to paint a picture, when I really figured this out was a few days ago in Act 2 when I fought Wasp Spiders for the first time (if you don't know what those are, they're poison-spitting spiders with lots of HP that attack in packs). The first time in there, I wiped faster than I had against
anything in the game prior to that. We're talking dead in 3 seconds kind of thing. Went back to my Manor, equipped nature resistance runes on all my characters, put lightning runes in all their weapons, and voila I was through the fight with little trouble in a party consisting of myself, Varric, Aveline and Merrill (i.e. no healer...).
Makes a big, big difference.
I would not, however, criticize anyone for lowering the difficulty setting on the boss fights...