I had to call it quits in that particular fight as well. I was so frustrated that I wasn't able to finish the game (nor able to leave Hall of Echoes and finish some other quests) that I ended up uninstalling the game (which I normally don't do with RPGs as I play them at least a second time around). Up to that point I was actually enjoying the game very much.
Mind you - I was playing on "easy" and still wasn't able to beat those final guys.

Cheers,
Rainer
I really laugh when people go i couldn't go back and do side quests. The game tells you that you won't be able to go back that this is it, and its not so hard to belive that you get to do whatever you want till you have to face a game changing aspect, and even then the GAME WARNS YOU do not proceed if you have unfinished business in so and so place. so do not blame the designers on it, you knew it was gonna happen before you entered. and if you didn't pay attention to your game that is not a design flaw that's a player flaw.
Yes the final battle is hard. but its not impossible to beat and honestly its fun that they gave you a challenge instead of here you go beat the boss and win the game in 2 seconds.
Last thought, if you can't beat them its not your build its your gear. you can go unstated and win everything *it won't be easy but you can do it* Also on a note above there is no such thing as a seamless game no matter how open world you think it is, you follow the main story and you beat the game you can take 40-200 hours to beat it but eventually you come to the ending by following the main story