This is an old trick that is intentionally left in the game. Priests can use Shadow Word: Death (SW:D) to remove a mage’s Polymorph spell. This is done by casting the SW:D at approximately the 50% mark on a mage that is casting Polymorph on you. So long as the mage does not die you will receive damage from your own SW:D immediately following the Polymorph spell. Since any damage will break the Polymorph spell the returned damage will pop you out of the Polymorph. Keep in mind that SW:D has a 12 second cooldown so that you can not break every Polymorph the mage casts but you can at least break the first one. If the mages casts polymorph again then you may trinket out of it. If the mage casts polymorph a 3rd time then it will only affect you for 2 seconds due to dimishing returns.

You must be aware the mage is casting the Polymorph spell on you and have SW:D ready. Here is a video demonstrating how to break a Polymorph with Shadow Word Death.