From the Super Mario Wiki
When the attack begins, Mario and Luigi jump into a cannon. A randomized sequence of them blast out into the far background, there being three of each Bro. in the sequence. The player must hit and in the same sequence Mario and Luigi came out as they land on the same enemy, and if successful, must do the same sequence backwards for maximum damage as they come down in the form of a large yet slowly shrinking Spin Jump the second time.
The move resembles the Cannonballer from Mario & Luigi: Partners in Time, and the poses the Mario Bros. perform when achieving an "Excellent" are borrowed from their level up poses in Mario & Luigi: Bowser's Inside Story.
Memorize the order that the shadows of Mario and Luigi launch into the sky. As they land, press the A Button for Mario and the B Button for Luigi. If you get it perfect, they'll descend in the opposite. Stay focused!
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