There are 5 ways:
1. Using the Dragon Fist.
2. Using the Final Kamehameha.
3. Hitting him with Wuuthrad.
4. Hitting him with Bloodskal Blade.
5. Using Lightning Storm.
You can still kill a Nord, or any other race, with Wuuthrad.
6. Using any method that succesfully hurts Miraak enough times until he dies.
He is actually a daedra.
Banish daedra actually damages him.
What do you think?