I'm not really a fan of the combat in this game, so I've been playing through on casual. I do have the ultimate edition though, and I want all the trophies, so I'll have to do those DLC bosses on nightmare. Now I'm thinking I'm not going to do the DLC on this playthrough (warrior), as I really didn't pay much attention to my build. But is there a consensus as to what the "best" class is for these challenging battles?
Two-handed Warrior is probably the easiest, as you basically just run in, draw aggro from everything with skills, then beat on everything till they die, while a Mage heals you. If you are also a Templar and use the right armors, you can also make yourself extremely resistant to magic attacks.
Mages and melee Rogues have a lot of tactical positioning to work out (particularly on higher difficulties for the Mages), and all other types of Warrior are outclassed by Two-handing.Archer rogues are pretty easy to play, but also probably contribute the least to the party (other than maybe archer and dual-wield warriors, which I've not really played with).
As far as optimal building and Nightmare difficulty, there is a consumable item in Awakening that lets you completely re-allocate all types of points on a character, and this item is purchasable en masse at a shop.