silver bullet

any simple but sweeping solution to a complex or virtually insurmountable problem: the legislation was no silver bullet for crime

See silver bullet in American Heritage Dictionary 4

noun
  1. An infallible means of attack or defense.
  2. A simple remedy for a difficult or intractable problem: “There is no single silver bullet or panacea that will solve all the problems of Bay Area schools” (San Francisco Chronicle).

Origin:

Origin: From the belief that werewolves could be killed with silver bullets

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