Further to Kiya's advice:
Shaman magic, nature, lightning to be specific:) My main character is my mage. She has about 7 in lightning now, I have the DK in the lead and I just walk along while she shoots off bolts, killing everything we pass. Then it's time for a little rest and more touring:)
Don't waste time putting anything into Fireball when you finally can get it. It is the only spell that backfires on you. You end up doing more harm to your party than to the enemies. I learned the hard way, putting in the points, and killing myself numerous times. Then I paid to get rid of the skill to get the points back to put in lightning.