Just completed game on nightmare, using rogue archer artificer, Blackwall, Iron Bull, and Vivienne. Couple notes...

Blackwall is unkillable, and the "guard on hit" ability breaks the game. Iron Bull works better as a "sword and shield" on nightmare difficulty. Artificer is also OP. Critical hits, throwing knives, and mines pretty much end battles within seconds.

Playing the original Origins game made me realize how much easier games have become.
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Yeah, Blackwall is untouchable, barely takes any damage and any S&S character with high guard once you build their skills is going to be the same, even on nightmare. Only the worst enemies will make a dent, like high level spells. Even demons are no match.

Artificer is nice for your PC but sucks on Varric. Mines don't hit a lot because of poor AI placement and to get into range, he has to get close enough that he takes loads of damage. He died a lot in my play through - all of my range attackers did - because they're dumb.
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