I was having issues in WC also with my 5 class setup. However, it was more due to me not having enough experience and control over my characters to control them. Entered WC on lvl 17 and got first 2(3 if you count kresh) bosses down quickly, however, getting stuck on the time it is taking to clear the rest of the instance.

Mage keeps running oom much before the other though. However, he also deals about 40% of the group's damage.