You have 2 choices:
1. All out offence: go with your best dps weapon and try to kill the mage before they kill you,
or,
2. Try to outlast them by using heal spells while they run themselves out of mana, so that you can then melee them to death at your leisure.
Either way, you need to be up in there face, and swinging your weapon - hopefully your melee hits will slow their casting down. Us your stun spell to interrupt a cast, and re-use it as often as you can.
Bear in mind that at level 22 you will get a new spell that gives every melee hit a chance to stun the opponent - that, I think, will be good for pvp against a caster.