silver, daedric, or enchanted weapons. i find a 'hit and run' strategy works best. rush up to it, hit it a few times really fast, then run away. it takes a couple second for it to cast it's drain spell. wait for it to materialize again (though you can hit it when it is pretty much invisible), and attack again.
rinse, repeat.
alternatively, if you have a lot of arrows to waste and a daedric, silver, or enchanted bow, a slow retreat works well. just keep moving backwards, preferibly in a circle pattern, and keep shooting until it dies. good damage on your bow is very important here.
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~ toodles
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