If you want an easy win for the badge, I recommend the storm mage. I tried all the other classes but all the rest at late levels seem to have a hard time. On the other hand, the storm mage has enough range for all the enemies, has stun if they come to close, can hit multiple enemies with one spell WHICH can be used again instanly to finish the job. And all of these from a safe distance. Just focus 2/3 on focus and the rest 1/3 on spell casting. Try to pay for talents points if possible and enchant his weapon when opportunity arrives.
*Note: Do not use electric spells when you are in water because they can damage/kill you. Learned it the bad way...
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