Lakitu Launch (multiplayer)
From the Super Mario Wiki
Lakitu Launch is a 1-on-1 minigame featured in New Super Mario Bros. While it shares the same name as the single-player version, this version is a completely different minigame altogether and can be played only by two players.
To play, both players use the stylus on the Nintendo DS's Touch Screen and must launch colored balls at Lakitus that fly across the screens. They will get 100 points for each shot, followed by 200, 400, 800, 1600, and 3200 points if they land multiple shots consecutively. Some Lakitus carry a Star, which is worth 1000 points. However, the points go to whoever's screen the Star is on, so the players should be careful to hit it only if it is on their screen.
Take aim at Lakitu with your trusty slingshot, and open fire! You score points if Lakitu drops a Star in your area. Be careful not to let Stars land in your opponent's territory, though!