I have the same issue.
I used Galek's Crusade Blade and 2 Skullsquires's Cold Grasp. I was at stage 5 and did it 3 times. No success on ether of them.
Once placed a curse on each of the 3 at the end, since I didn't know who was it.
Once I found out who was it - I placed it on the Guardian.
And once bested him while the curse was still on him.
Beat him each of the times!