From the Super Mario Wiki
Mighty Meteor is a Special Attack that appears in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. This attack costs eight SP to use in the original, and twenty BP (twenty-two BP on Easy) in the remake. It is obtained from Dr. Toadley's intern in Toadley Clinic after the three Star Cures combine to make the Miracle Cure. After the Miracle Cure cures all infected Toads from the blorbs from the Mushroom Kingdom and before Mario and Luigi leave the clinic, the intern will give Mario and Luigi all 10 Toadley 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 and hold down the corresponding button ( for Mario, for Luigi) as it falls to reel back their hammer, swinging it forward once the button is released. If the brother successfully hits the meteorite with their hammer, it crashes into the enemy to inflict damage and possibly lower the enemy's attack power. The meteorite then goes back to opposite brother, and the attack continues. If the brother fails to hit the meteorite before it hits the ground, the attack stops.
After successfully hitting the meteorite enough times, it 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. The remake also includes the Ultrasyrup Jar, Refreshing Herb and Max Syrup Jar items. This effect occurs even during boss fights the Gauntlet, allowing Mario and Luigi to effectively use more items than they are allowed, and since Star Candy fully replenishes SP, as well as HP, it is possible to attack and gain an item every turn, stopping only to regain SP with consumable items.
In Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, the meteor is now a 3D model that lacks the "?" icon and explodes into confetti when the attack ends successfully. The Bros. Attack also plays the last few notes of the classic level complete theme from the original Super Mario Bros. when it reveals an item.
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