Will-O-Wisps, Slippery little f***s Options

[Parkillous]

*note this post will spoil the Leweyin (sp?) mage guild quest*
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Aight ive gotten to the tombs to get the old hags amulet and BAM im dying from something... then i find its this wisp thing that doesnt want to take a hit from my sword so i use flash bolt which effects it but very mildly.
I send it on a wild goose chase around the chamber then i get the amulet and make a run when BAM the knight who stole the hags amulet comes and challenges me... i continue to run because the wisp f***ed me up and eventually get back to the mages guild but uh oh she wants to believe that the amulet im carrying doesnt exist until ive officially "completed" the quest (im assuming) by killing the knight and thus I will also have to face the little wisp when i go back to the fort...

which brings me to my question, how do I kill the Wisp?????????????