Like Turel, he is invulnerable except for certain moments.
Dodge him when he dive bombs you, then get in close and slash
at him. You can also attack him after he has shot the orange tornado
at you. Attack whenever he is down on his knees.
Build up your Reaver spell and be ready to cast a spell (Spirit
Reaver recommended). Do not use the spell immediately. Instead,
wait until he attacks with something like the orange tornado and
you are somewhat close to him. When he does the attack, immediately
cast the Reaver spell. As he goes to his knees he should take
some damage from the spell. you Also attack him at this point
as much as possible to inflict more damage. It is recommended
that you attack him until just before he starts taking no damage
from your attack again. Then, repeat the process. This takes time,
but helps to conserve some health and inflict quite a bit of damage
at the same time.