Why didn't tough on this battle? Two uber mages (except for the fact that Sauron isn`t exactly a mage..), immortal alike, booth fight over the control of booth worlds, Nirn and Arda. They will fight with everything they got.
-Miraak posses a legendary dragon as ally.
-Sauron posses the One Ring.
Boot hdetaisl above can be removed if the fight seems to unfair for u.