Mage

If you can survive the opening barrage against a mage, then the mage is dead. 

There are two important things to remember when fighting a mage:

  1. Stay out of Shadow Form. You must be able to heal yourself.
  2. Try to cast only instants except when the mage is feared.
     

If you happen to sneak up on the mage, open with a Blast.  Drop renew and shield on yourself, then one/both your DOTs on the mage, followed by a series of dispels, since Mages have several magical buffs. Then close with the mage and fear him.  When he starts running around either Blast him, or heal yourself, then start flaying him again. You do not have to damage the mage to death.  A Blast and your two DOTs will kill them given enough time. Try to save your silence for a fireball or a polymorph.

At some point during the fight the mage will silence you.  This is your opportunity to heal your self back up to full life.

Most mages will either try and instant blast you to death or polymorph you then build a pyroblast.  If they start with a poly, you will start the battle at about 2/3s health then receive up to 3 instants damage spells almost immediately. You need to have your Shield up first thing when you come out of Poly. The mage will try to blink away from combat when he is low on health, or sheep you and bandage.  Be patient, and ensure that the mage always is infected with pain.  The mage’s death is inevitable, you simply must survive.
 

Finally, be wary of your life. A mage has two instant cast damage spells, and many have a talent (PoM) that can make any of their spells instant.  If you drop below half life, expect to recieve two or three quick blasts

A shadow priest should be able to kill a mage the majority of the time.
 

Mages are also a class that Holy Priests can easily kill.  In this case, blast and DOT the mage as soon as possible, then concentrate on healing yourself.  Your healing spells are much more efficient than their damage spells, so he will be out of mana before you.  Just ensure that your DOT stays on the mage, and you keep your life nearly full.