Mage is good against everyone just hard to master. Priest can also be good and I saw shadow priest with full wratful winning duels against full pvp ret paladin. He won like 6 duels against him and I was impressed. Warlock is decent too, I'd say affliction is to go because of heals you get, but destro is also good for fast burning damage.

Now all casters get one mana and that's cloth armor and hard survivability against Shadowmourne and death knights/Warriors mainly because of all slows they can put on you and immobilizing effects. Of course, you have fade/blink and other things to help you but you gotta be really careful against them.