The mage is incredable with damage, but if you choose it, choose your other person to be the shield person. He can easily defend and support the mage while the mage destroys the enemys. Dont just count out the mage.
So, persoally, out of all of the deffence quests, i couldnt do any of the second to last category, because there would always just be too many. The impossible one on the other hand was one of the easiest defence quests there were.