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My attack spell seems can't hurt it.
also I can't use melee attack ,I'm a magic build.

You have to talk to the Farmer in the south of the cave. He'll give u the cyclops blade. If you try to kill the cyclops than it still wont hurt him (but u have to try). After that you must go to the echantress who lives in a house, southwest of the cave. She will enchant your blade and you'll be able to kill the king.