I gave up Hard for the boss battles at end of chapters 1&2. And I gave up at Hard the battle BEFORE what's probably the last Black Book, at least the fourth book and I don't want know yet what's the battle after to take the book, lol well I wanted keep it for a replay. The black book quest is over now in chapter 3, but the cavern entrance still here, I haven't bother yet enter here, prefer not know what's there in chapter 3, lol.
I don't remember at all the battle you mention. Perhaps you could refresh my memory with a quest name and a location in a spoil tag?
For some fights I beat them on Hard but applied some DAO tactics of isolation. It depends a lot of your spells and talents but I'm surprised you can have at least the first move. In similar cases (there's more than one dialog that can be concluded by a fight start) it's not fully normal your mage get all the agro on start. But as my party always have two mages I possibly never quoted it well. So a first try is perhaps take with you another mage to see.
Other than than try all other member attack and your mage only retreat, and one fighter could try some quick AOE attack. Then it depends a lot of your spells and talents.
Also even if your mage is knockdown at beginning of fight it's not the end of the fight, have all other members support him for this fight start, Rogue have some AEO control, 2H have some quick AOE attack to win few time for your mage, possibly a fighter or a rogue could have some agro skill that could help. Also another mage could help your mage, there's many different spells for that.
For me the Hard level is still better than Nightmare was at my first play in DAO. For more casual fights, I often still need put some concentration on what I'm doing and how it turns otherwise I could lost it. For the quests significant fights, like after a dialog, I continue lost some, but it's rare I have to replay one more than once. For sure when there's new type of enemies or new special combination of opponents the elements of surprise and discovery increase significantly the chance to lost. It's clear that all those modern RPG need more enemy diversity, I don't mean graphics, but for good gameplay this involves also new graphics.
I suspect that when I'll replay, a playable Nightmare mode will be missing. I'll use a mod removing Friendly Fire in Nightmare but the comments about this difficulty level let think it won't work very well. Well I'm not much confident in players comments about this game, too many are so far from my feeling. But I don't remember any good comment about this difficulty mode, perhaps without FF it will be roughly ok, at least I didn't read much comment about that.
About that Nightmare mode involving long fights, I smile when I read ton of comment complaining fights can be too long and I'm sure the same complain DA2 isn't Hardcore enough. They should have played old games. There was some memorable endurance fights in Pool of Radiance, quite more than 30mn but well it's turn based.