Assume 1 day of prep time for Batman and Vader at peak power.
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Well Batman doesn't kill... But Vader doesn't use prep time.
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I'd say Vader but he went down to -blam!-ing Boba Fett once.
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Batman with prep time
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Vader
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I believe I've already seen that done already and Vader killed Bats by using the Force to throw a Batwing-type flying vehicle into a wall and causing it to explode.