Hi!
I just killed him on hard today.
And to be honest, it was not a walk in the park. Same as you I thought the way to him was very easy (I did not even think it was the end of the game since there are content in the master dungeon that is way harder). But man, that guy was rough. The thing that helped me to get into a good position was to have Edér Grab him instantly into melee and stun him, so my "squishy" guys could get into a good position. But when the fight was over I had 0 spells left and Edér was down to his last few health points.
But he does the big AOE a few more times in the battle aswell, so remember to keep your guys fairly high.
Hope this helped!