For many of you it might have been a parkstroll, for me it has been painful to say the least.
I tried with my level 61 necromage at Legendary difficulty, after beating all the main quests of vanilla Skyrim plus dlc (at lower difficulties, from adept to Master).
I tried countless times before beating it, not without the final destruction rush I managed to do, holding my breath
During the various stages of preparation, consisting in the search of the best equipment, 3 revered dragon attacks happened, all at Raven Rock. The third one saw me running away ( I had run out of patience) trying to fast travel to karstaag's battleground.
At last I managed. At last. But I must admit that one particular power given by a Black Book helped me a lot in the final attack.