This strategy is hands down the best possible way to play a Shadow Priest in arenas, and applies to all team sizes and brackets.
Step 1: Mount up and run straight into your opponent.
Step 2: Use Psychic Scream. Throw a SW:P on someone, too, if you get the chance.
Step 3: Get stunned and dropped to 50% health in the half second it takes your opponents to react to the fear, then hit Dispersion. If you're really good, you can even Disperse before they drop you to 50%!
Step 4: Jump around in circles until Dispersion wears off.
Step 5: Throw up a Shield, Prayer of Mending, and (if you're really lucky) Renew.
Step 6: Proceed to die in the most horrible manner possible. If you were lucky enough to get a Renew off, don't expect to survive long enough for it to tick.
If you can follow all of these steps perfectly, you're well on your way to 1600.