
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! 