From the Super Mario Wiki
Mighty Meteor is a Special Attack that appears in Mario & Luigi: Bowser's Inside Story. It is obtained from Doctor Toadley after the 3 Star Cures combine to make the Miracle Cure. After Mario and Luigi destroy the Dark Star's blockade, Dr. Toadley's intern will give Mario and Luigi all 10 Clinic Attack Pieces required to use this move.
When using this attack, Mario or Luigi sends a huge meteorite up into the sky. The player must control one of the brothers with the D-Pad and press the corresponding button when it falls. If the brother successfully pulls off this command, the meteorite crashes into the enemy, thereby inflicting damage, then going back to the Bros. and the attack continues. If the brother fails, the attack stops. This attack costs eight SP to use. This attack is very similar to the Smash Egg from Mario & Luigi: Partners in Time.
After a successful attack, the meteor bursts open and the Bros. receive a random item. The items are either an Ultra Mushroom, a Max Nut, a Star Candy, or a 1-Up Deluxe. This effect occurs even during the Gauntlet fights, allowing Mario and Luigi to effectively use more items than they're allowed, and since Star Candy replenishes SP fully, with some luck it is possible to attack and gain an item every turn, stopping only to refill SP.
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