I hate mages too, but poisons that damage health or magic seem to be pretty effective against them. If they are with archers and/or warriors, I poison the mage with an arrow and rush with my sword, making killing him my first priority before I move to another target. Groups of mages are a little harder and I generally try to keep my distance and take pot shots with poisoned arrows or try to lead them into a small corridor, hallway, or doorway - anywhere they have to come single file - and just slice up the one in the front of the line 'til he drops then move on to the guy behind him. That way instead of 3 or 4 versus 1, it's more like a series of 1 on 1 fights (which should give you the advantage, most mages are squishy) because usually only the guy in the front of the line can hit you with a spell.
On my next play through it would be fun to get into destruction and fight fire with fire, but as a warrior that's the method that works best for me.