Now as far as builds. Here is what I would suggest.
My first playthrough on Nightmare (I always do Nightmare first in case I end up not wanting to play a second playthrough, I get the achievement upfront) was as a Qunari Archer. Basically you want to put your points into the Archery Tree focusing on any abilities that get you Dex or Cunning. You can put the rest of your points in whatever other tree you want. The minute you get to Skyhold, you want to unlock the War Table mission that gives you specializations and I highly recommend the Assassin specializations. Once you get it, respect your Archer to put max out the Assassin tree (except for the Sleeping Bomb one) first and then Archery. Doesn't matter what else you put your points into. For high level enemies (dragons, bosses) you basically throw Mark of Death then Hidden blades and then whatever other Archery abilities do big damage. You need to watch the top right corner for when the Mark of Death starts flashing as that is the cue that it is about to run out. You basically want to trigger it before it runs out as you do bonus damage. This method once you have tier 3 Weapons basically gives you 25-50k in damage. I pretty much can kill dragons in a couple minutes using this strategy and can kill Giants and other high level enemies in one go. Your strategy ultimately becomes making sure the rest of the party keeps any low level goons off your archer so that the archer can focus on taking down the big game. I normally run with at least 2 mages with my Archer for their barriers and crowd control abilities and one warrior to keep people form coming after my archer because he fucks people up so bad that everyone basically wants to come after the dude. You also want to keep Stealth in reserve in the event someone does come after your archer so that you can lose them. As far as weapons and gear, you need to get to Hissing Wastes as I said above and do the Tomb of Fariel. It gives you the Tier 3 Bow and if you have killed enough dragons then you can get the best bow in the game by using Dragon Bone. You also want to get the Fade Touch material that gives you a 10% chance of using Hidden Blades because that just gives you another chance at getting Hidden Blades which makes your Archer a Beast at killing shit. As far as Armor and accessories, you want to focus on Critical Damage and Chance and want to be around 150% for CD and 20-30% for CC at least.
My second playthrough was as a Two Handed Dwarf and I started on normal because nightmare is just a drag on the low levels and I already had the achievement from my Archer above. Basically you want to start by putting points into the War Cry side of the tree ending in Unbowed so you can get Guard. Otherwise, dude will get killed quickly. Then you want to get the Reaver Specialization and fill out that tree. Your combo for this will be Ring of Pain, followed by Devour followed by spamming Dragon Rage. You only use this combo on Dragons or other high level enemies. Otherwise, you can take down normal goons with just other abilities and Ring of Pain because Dragon Rage reduces your health. You also want to get the Inqusition perk that gives you extra health potions as soon as possible because you will need them with Dragon Rage. This dude doesn't kill things as quickly as the archer but he's still pretty much quick and the bonus is that with the Guard you gave him and 12 potions, you can pretty much solo Dragons with him by spamming Dragon Rage. You also want to give him hidden blades on his weapon as well as either hit on guard or health on kill on his armor so that you get bonus damage and guard or health regen. Finally Sulevin Blade (from Cradle of Sulevin) is the weapon of choice (it has 47 strength) and is better than the Crafted two handed weapons unlike with the Archer although the advantage of the crafted weapons is you can use them without any regard to having to be a certain level. Once your strength is around 80 and you constitution around 25-30, you want to pump the rest of the points into gear and accessories that give you CD and CC so you can get the bonus to damage. Finally, I run with a Dwarf because the Legion of the Dead Armor that you get from Tomb of Fariel in Hissing Wastes is the best heavy armor in the game and can only be worn by Dwarves. One other point. The LOD Armor which is heavy armor can also be used by Varric but only if you put build it with Silverite which you get from emprise du lion. Otherwise, it is usually restricted for him because it is heavy armor and he is usually restricted to medium armor.
Finally, my current play through is with a Mage. For this dude I focus on the Spirit Tree for barriers and the Electricity Tree because it offers the best DPS. The other trees (Cold, and Fire) are more for crowd control which I use my other Mages for. I pick the Knight Enchanter specialization because as I noted with Vivienne above, it gives you unlimited barriers so I basically use the electricity abilities when I am at range and then if anyone gets close I spam Spirit Blade for unlimited barriers. I also use Spirit Blade on Dragons as I can pretty much solo them. The only downside is the DPS for the Mage is much lower than the Warrior or Archer so it takes longer but again I pretty much have limited risk of dying with the barrier regen that Spirit Blade offers. I usually pick an Elf for a mage because the Keeper Robes (also from Tomb of Fariel) is the best light armor in the game and can only be worn by Elves. I usually outfit my staff with Hidden Blades again for bonus damage and my armor with Guard just to add another layer of protection for my Mage. I then fill out my party with whoever the hell I want because KE dude pretty much can survive anything and so isn't as dependent on other characters to keep goons off him like the Archer and isn't at as much risk of losing Barriers/Guard as the Two Handed Warrior.
