Going back a bit to the original topic, what about Feanor? I doubt anyone in Middle Earth had a stronger will than he, but he was destroyed in the height of his glory by mere Balrogs

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As we hear, the will was not overthrown, but he was killed by phisycal means, namely a whip from another Balrog. If we go by the hypothesis that if you are strong in will you are inpregnable to physical hurt, how is this possible?