To be an effective mage you have to use your stuns and your shield well. For example, a good strategy is to put up your shield, stun the enemy and run through the enemy's warriors and attack the rogues in the back. They usually are not ready for it and if you know what you're doing you can take out a rogue or two before the stun wears off the warriors. Then you can turn back towards the warriors and help the group take them down.