My general impressions:
- Nightmare difficulty is, luckily, significantly more challenging than the corresponding difficulty in DAO (not to mention DAA).
- However, this is in part due to the fact party tactics cannot be really turned off in DA2. You have to literally babysit your party members if you want to be remotely successful on Nightmare.
- Companions are surprisingly useless, with the exception of both mages and Aveline (and even those leave a lot to be desired). To be fair, Isabela has a lot of potential thanks to her auto-crit AoE talent. It's just takes a lot of time to unlock the aforementioned potential.
- Archery talents are worthless even on the protagonist, and don't get me started on Varric and Sebastian :( Auto-attack archer builds are nearly as viable as they were in DAO/DAA, though.
- Rogue class has two protagonist-only uber-specializations (Assassin and Shadow), and a whole bunch of crappy talent trees (though, as mentioned, Isabela's Swashbuckler is not hopeless). Mages and warriors are far better balanced.
The ideal party thing. A word of warning: it's a subjective ideal we are talking about, yes? :) Okay, so here's my take: 1. Protagonist: two-handed warrior, Reaver spec. Key talents: Cleave+Claymore, Might+Muscle, Blood Frenzy, Sacrificial Frenzy, Whirlwind+Tornado+Cyclone, Elemental Aegis (upgrades not necessary). Stats: 50/50 Str/Cun. Yes, Cunning, not Constitution. Role: he is the guy who kills things. 2. Aveline. Key talents: Bodyguard+Elite Bodyguard, Battle Synergy+Hero's Synergy, Serve And Protect (approval talent, make her a friend). Stats: 100% Con. Role: prevents the Reaver Hawke (who will be typically at 10% hp) from being killed by stray arrows/attacks by blocking 85% damage done to him. 3-4. Merrill & Anders. Key talents: Cone of Cold+Deep Freeze, Elemental Mastery, Elemental Weapons, Heroic Aura+upgrade, Mind Blast+upgrade. Stats: 100% Mag. Role: they make the opponents brittle, then run for their lives. That's about it. Party buffs are nice, but hardly crucial.
Typical engagement flowchart vs multiple opponents: Make sure protagonist has 100% cold resistance (easy to achieve with rune-enhanced Boots of Frozen Wastes + Elemental Aegis). Put Aveline in the rear (make sure she has Elite Bodyguard active on the protagonist), she is protagonist's hp pool, not much else. Buff the protagonist with Claymore, engage enemies, lose a lot of life, activate Sacrificial Frenzy. Both mages should cast Cone of Cold in the meanwhile (the protagonist will resist it anyway). The typical outcome is ~80-90% of the crowd will become brittle. Immediately fall back with the mages, unless you are suicidal. Position yourself properly (usually you are already surrounded), Whirlwind > battle won, congrats. Finish off the remaining opponents (if any) leisurely. At level 13, with decent weapon and CriticalRange property about 100, Whirlwind will do 7-8k damage on brittle opponents.
An alternative party, in certain ways superior to this, consists of Fenris (make sure he's a rival for a +10 CriticalRange) + protagonist mage + Merrill + Anders. The same principles apply. Fenris' damage output will be somewhat lower (he will still one-hit practically everyone with Whirlwind), but 3 Cones of Cold will make 100% of opponents brittle in most battles, which is safer and more convenient.