For mages it can be painful while leveling to take multiple mob. But it will get better as you handle the class better and get more new spells: combination of Blink, the Ice Cone thinggy (sorry, never played a mage myself but leveled one of my toon mainly with a mage buddy) and all will make it easier. Depends also where did you put your talents, I think Frost is better for leveling for the higher survivability and all the slowing spells

Pallies after level 32 (if I remember properly) are ultimate king of taking on multiple mobs.
With a pally you end up letting the mobs running away go so they bring their friends along

my friend aoe grinds his mage from level 30......