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Charges up turn one, and then attacks turn two.
Like all moves that take a turn to charge up, Skull Bash is not a good competitive move. Unlike other charge-up moves like Fly or Dig, Skull Bash does not allow the user to avoid attacks. Like all other charge-up moves, during the charge-up turn, the opponent can easily switch to a counter who can easily absorb the Skull Bash like Snorlax, giving the opponent momentum. It's a very bad move that should never be used.
Body Slam is a more powerful alternative which boasts a 30% paralyze chance.
Double-Edge is a much stronger option with recoil.