
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! 
What gives with this challenge? I had scale hacker from gromil decrease Def and vine trap from sorceress decrease Def both on the guardian. I still can't complete the challenge.
And this is on the boss in Stage 5 of the Force Keep? can you take a screenshot of the challenge and also ensure you're meeting all requirements? There's no known bug with this.