Go to the Gold Shop, located at Driftwood Shore. Buy and equip two Angel Bangle items (one for each brother). With the Casual Bros. rank-up bonus in place (or with the 50% off gear and Gear Slot +1), you should now be able to use infinite Luiginary attacks.
Jukebox
Beat the game.
Picking on Luigi
On the title screen, you can tap on Luigi's head to interact with him, instead of beginning your game. You can pull Luigi's mustache to tilt the game's logo, or rub his nose to make him sneeze (causing the logo to flip). Tap on his hat and the logo will instead go dark.