What's a word or phrase that means 'take advantage of someone's psychological weaknesses'?

Solution 1:

gaslight

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Oxford dictionaries

verb: Manipulate (someone) by psychological means into questioning their own sanity:

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dictionary.com definition

  1. to cause (a person) to doubt his or her sanity through the use of psychological manipulation:

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I think that's the closest I'm gonna get to what I want..

Solution 2:

Try play to a weakness. Here's part of a review of a book about Simón Bolívar, who led the campaigns in the 1820s to oust the Spanish from their South American colonies:

Though the author describes many battles, it is all too much like a game. As far as I could tell Bolívar’s main tactic was to attack head-on. There is little indication he studied the terrain, disposed his forces according to it, tried to understand his opponents’ mind and play to a weakness, as did Robert Lee.