I was curious about defeating Miraak and read that he's supposed to be level 150. Knowing that scared the crap out of me, but my curiosity sent me to fight him anyway. At level 40, I kicked his butt and took barely any damage. He was almost too easy and Alduin was more of a pain in the butt than he was! But only because you basically have to fight him twice. I even had the game level on a tougher setting.
Miraak levels with you hes always 1.1 times your level, so he was level 44 when you fought him. Level 150 is his absolute max level, even if your level 160 he'll only be 150. Also his minimum level is 35