So with the dragonborn DLC you are just doomed to serve hermaeus mora no matter what you try to do in that main quest line. This seems pretty stupid to me. I was trying to search for a way to possibly kill him or go against him, but it seems it is hard coded into the game that you have to serve him in order to complete the quest line. No choice. Again, stupid. Now I was reading this thread: http://www.gamespot.com/the-elder-scrolls-v-skyrim/forum/level-81-vs-that-hermaeus-mora-dufus-possible-spoilers-65788356 and people were saying things like "he can't be killed, he's immortal" blah blah blah and I remembered something. The Elder Scrolls. Now it isn't a verified fact that he can't be killed and Mora himself even comments that the dragonborn could be a 'worthy opponent or servant'. This got me to thinking about how one could possibly use an Elder Scroll to defeat him. Now, as I understand it, the scrolls themselves are supposedly left over fragments of creation, created by beings more powerful than the daedric princes themselves. You also will not find a single elder scroll in any of Mora's realm sections. I am willing to bet that not even he can read them/understand them so easily. With this in mind, I would use the elder scroll that was used to 'defeat' alduin on Hermaeus Mora, only in reverse of what they did to Alduin. I would send Mora far into the past so that he would have to live through all the ages to get back to the time which I sent him into the past - at which point I would send him into the past (again from his point of view). This would create an infinite loop of sending him into the past at a certain point in time so that he would no longer be able to progress beyond this point in time and would therefor cease to exist past that certain point in time for all practical purposes. I win. Suck it H Mora. ...bitch signed, Dragonborn
This isn't to say that I think my character couldn't best him in straight-up single combat, however. I am level 82, and with the gear and potions I have, I can reach a maximum of about 900 damage with my bow (not counting sneak attack or critical. Firing arrows quickly and utilizing dragon aspect, and the fact that I also have about 80% magic resistance and over 600 hp - I am willing to bet that I could kill the bastard in single combat. Oh, I also have legendary(2) Archery 100/100, legendary(1) light armor 100/100 with every perk taken in both of those trees. I have one shot a revered dragon before with a sneak attack critical.