I have over 100% Spell Absorption, yet I'M STILL GETTING KILLED BY HIS/HER MAGIC!!!

This is absolutely ridiculous. How is this possible? I'm whaling on this thing for minutes, and its health bar is only down to like 25% I've tried all different sorts of weapons, but NOTHING WORKS.

How the Foxtrot Unicorn Charlie Kilo do you off this bi tch?

I should add:

I'm level 39.
I have 100 in longsword with bound sword
My strength for this battle is around 500 with potion use.
I'm gulping custom restore health potions like they're going out of style (something I haven't done since level < 10)
I've tried a daedric dai-katana with +10 fire damage for 4 seconds.