I imagine that would be exactly the sort of weakness that could really beat the tar out of a fiend like that!  I'm reminded of Robert E. Howard's<em>Skulls in the Stars</em>, wherein the much beloved Solomon Kane encounters a completely intangible yet murderous shade.  To summarize, Kane fought his untouchable foe with such fury and unrelenting resolve that the shade began to fear<em>him</em>, whereupon Kane<em>made</em>the ghost solid and grappled with it before flinging the thing back into the marsh from whence it came.