Instructions
Find a Will-o-the-Wisp and select the spell "Weakness to Fire." If your stats are not high enough to use Destruction spells, equip either a silver or fire-elemental enchanted weapon. Iron, steel, glass, black and other materials will not deal damage to the Wisp.
Hit the Wisp with "Weakness to Fire."
Select "Flame Bolt" or another on-target fire spell and hit the Wisp with it repeatedly. If you do not have Destruction spells, alternate between attacking with your silver or magical (preferably fire-elemental) weapon and running -- Wisps can deal large amounts of damage and you will probably need to regroup and heal if you are lower-level.