When Mystic Reversal is used in response to Cull the Weak, does the player casting Mystic Reversal get to choose new targets for both players, or is it wasted?

Ruling: Mystic Reversal would be wasted when used on Cull the Weak because Cull the Weak doesn't involve making choices or selecting targets. The result would be the same as if Cull the Weak resolved normally: each player kills one of their units.