One of the Dragon Age issues is the highest difficulty level called Nightmare. Even though it should be better balanced and optimized, it is still unchanged what makes many players unhappy. The enemies on this difficulty level do a lot of damage and have additional resistances what makes the fight an ordeal rather than challenge. That is why one of the Dragon Age fans asked whether the same situation will be in Dragon Age III: Inquisition.
Please, this time around bother with balancing Nightmare instead of bloating enemy HP & Damage and throwing immunities in. Fighting half an hour the same creature is not funny (and who tested the duel with Arishok?). Maybe having enemy powers and tactics depend on difficulty level?
Arishok is a situation where I realized that even as "Tech QA" I should still find a way (somewhow) to play the game. I didn't get to that part until after the game was submitted to Cert and I wholeheartedly agree that it's poorly done as a 1-on-1 fight.
As for Nightmare, the unfortunate thing is that Nightmare targets a particularly niche crowd. It's a lot more valuable to have people making sure normal difficulty is working, and less so for Nightmare. Which sucks to hear if you're a nightmare fan.
Now we could do things differently with Nightmare than we did with DA2. What you suggest would indicate that the creature's level could be bumped up (this would unlock other skills, on top of providing additional resources). This works regardless of level scaling or not (with no level scaling, we bump the level up. With level scaling, we still bump the level up).
Changing tactics is muuuuuuuuuuch harder. This works in a game like Chess, because all difficulty levels do in a board game is limit the search space (they don't see as many moves into the future). Having actual AI routines that only come into play for Nightmare difficult would not be a trivial thing to do. Furthermore, if those AI routines are interesting and make the AI behave in a more plausible way, there's probably a value case for such routines to exist for all difficulty levels.''