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Deals damage and has a 50% chance to lower the target's Special Attack by one stage.
Mist Ball is a move exclusive to Latias, and while the 50% chance to lower the opponent's Special Attack is useful, the moves Psychic or Psyshock are preferred for their power. Most of Latias's targets are not affected much by a Special Attack drop either, as most of them focus on the defenses.
Psychic is a more powerful option.
Psyshock is a more powerful option that hits the opponent's Defense stat.