Well after playing through with nightmare I think the best party is a tank, two mages and a ranger. Tank draws aggro with taunt and can take a lot of hits, mages provide crowd control, healing, party protection, damage and resurrection, ranger opens locks, does good ranged damage and has a pet that can do damage, off tank and provide some extra abilities. Two mages are better than one because some of the combos like glyph explosion and storm of the century are just so much quicker to pull off with 2 mages.
For both mages I think glyph line (to repulsion), force field line, cold line (to cone) and basic tier 1 heal are all must haves. For morrigan sleep line and hex line are good because she already starts with spells in those areas, for wynne healing line and haste line are good choices for the same reason. Make sure one of your mages goes down the mana drain line for mana clash (which insta kills most mages) and spell might (for storm of century). The tier 1 direct damage spells are worth getting for all your mages too for the damage.
Your ranger (probably leliana, though she won't be able to get ranger till lvl 14) is better off going dex than cun, get enough cun to max lockpick but other than that I reckon dex is just better. In a 2 mage party you won't really see the benefits of her aura as much and dex gives you great damage, great attack rating and great defence, which is useful because she will probably be your most vulnerable party member since she doesn't have protective AOE spells like mind blast or glyph of repulsion.
Tank is pretty self explanatory - max shield/stoneheart skills and get taunt and threaten sustainable ASAP and you'll never have a problem keeping agro.
Strategy is almost 90% the same in any fight: main tank taunts and grabs aggro, mages use mana clash to take out enemy mages, crushing prisons/glyphs of paralysis/force field to take elites out of the fight, or sleep/fireball/paralysis explosion/cone of cold to crowd control large groups. Ranger and two mages are set to auto attack/autocast direct damage spells on the enemy with lowest health, mages are set to auto heal anyone below 75% health, I have group heal on manual so it's always there if things get dicey, force fields can also be used in an emergency too, and I'll frequently use this on the main tank when he's surrounded and taking a beating.
If you get the jump on enemies you can use glyphs of repulsion to block choke points and cast storm of the century or another storm spell - I actually like blizzard and tempest combo without using storm of century for a lot of areas because it doesn't have as big a radius but still does great damage.