Dragon Age II - Nightmare difficulty?

I'm starting DAII on nightmare right now. I've beaten the game 3 times on casual so now I'm looking for more of a challenge. Since I scarcely understand how to put together a good party, could someone help me? I have a feeling I'll need to actually care about my party this time around ._.

So, my character is a mage. I haven't leveled her up or given her any skills or anything yet cause I'm afraid I'll screw it up... What would be a good build/skill set for her?

As for the rest of my party, I've always used Fenris Anders and Varric. (They're my 3 favourite characters :X) I'd be willing to change it up in any way, and I would also need help with their builds and skills too. People say using Aveline is better than Fenris because she makes a better tank, but I always found that she dealt very little damage in comparision to Fen, who makes a good tank too. Which one should I use and why?

And I really don't know who is better. Would you suggest I use Merril or Anders? I like Anders cause he has the party heal skill where as Merril doesn't, and since I'm already a mage it would be excessive to have 3 mages in my party. So, which one of them would be better?

I think Varric is great for long range attacks cause he deals a lot of damage, but if you disagree please let me know about that too.

Any other tips would be extremely helpful; I don't want to try and beat any game completely unprepared. Especially this one!Dragon Age II - Nightmare difficulty?
The only way to get through nightmare difficulty is with cross-class combos. That means you need one member for each of the on hit effects, brittle, staggered, and disoriented. Mages can leave enemies brittle, warriors staggered, and rogues disoriented. I haven't gotten used to disoriented yet so only play on hard. You need at least one rogue, one warrior and one mage. You yourself should invest early into the ice spells to take advantage of the brittle effect. For a rogue, i find that Isabella and dual weapons have more opportunities to apply on hit effects, as archers lance does massive amounts of damage to brittle targets but is hard to line up, and exploding arrow for me has the same problem. Of course these could probably be solved with a simple glyph of paralyzation. As far as warriors go, Aveline will help you tank and set up lots of attacks, but will rarely leave enemies staggered so Fenris may be a better option. Play around with mages, rogues and warriors until you find a combination that works for you.