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My antagonist is a "strategic mastermind", similar to the one discussed in How to prevent seeming like a Marty Stu-ish villain is cheating? Proceeding from this kind of antagonist, my question is how can such a "genius" be defeated? How can I make the antagonist not accounting for something, come off as understandable?
I have often seen this problem being solved by handing the idiot ball to the antagonist - that is, making the antagonist miss a glaring problem they should have seen, making him suddenly act like an idiot - out of character compared to the way they have been previously described.
Are there other solutions - solutions that make sense, and do not break the antagonist's "mastermind" characterisation?