I had seen video of Sah defeating him, which gave me the heads up about what to expect.
How I did it.....
Using a legendary smithed glass bow (Dmg 322), glass arrows, and three strong potions of fortify marksman.
If you run in from the outside door and go half way to his throne, stop and turn right. There is a pile of rocks that Karstaag can not get you upon.
So, go to the throne, place the skull and run to the rock pile...climb up a ways.
Karstaag hangs around near the exit door. Just take the fortify marksman potions and pump 48 glass arrows into him.
Victory!
Adella on his throne
https://i.imgur.com/dhPSaTu.jpg
Of course taking him down sword and board would be a whole other story