You know who’s tired of this confusion? This guy. (If there was ever a bird to know his Shakespeare, this is the one.)
Perhaps with birds of prey, you might have a “fowl swoop.” Perhaps with gangly young basketball players, you might have a “foul swoop.” But when you’re looking for what to call a sudden, swift action, “fell swoop” is the correct form. How many of you are writing this one right?