Think you should ask morganti, either he is shit at arena's (hey morg! ./wave hows the pain?) or the all priest way is a bad thing. Could be a gear issue since morganti seems to be undergeared but he is having a bad time in arena's. Getting a lot of loses thrown at his feet at the moment. He is even levelling an all shammy team at this very moment. But again for details you should ask him about his results.

edit: actually he already did: Some Concerns with Five Shadow Priests