Mage aren't beatable.
Ice Block shedding our DoTs and (possibly) intercepting a Mind Blast + SW
combo is too crippling.
Then there's also the whole 'Lol I just spellstole your PW:S' thing.
Not to mention the countless abilities a Mage has to reset the fight.
Any Shadow Priest will be extremely hard pressed to get a Mage down before either OOMing or dieing. Killing a good Mage just isn't possible.
Related note, my UI doesn't show my keybindings.
To the person above, the Priest has to stand still for 3 seconds to apply DoTs; 4.5 seconds if you include VE. If you freeze it while it's reapplying DoTs, it's stuck in your LoS for 4.5 (6 with VE) seconds. Even if it spams dispel on itself, it's not guaranteed that it will get the freeze on it's first cast. LoSing a Mage for prolonged periods of time
is not possible unless the Mage is OOM or terrible. Even spamming dispels, you aren't guaranteed to remove the snares and roots. Deep Freeze is another issue, as well.
No Mage will blow CS on a Shadow spell, unless it's bad. It will save CS for once you're low or attempting to heal.
Shielding yourself is dumb. It will be spell stolen, period.