From the Super Mario Wiki
Mario Boo Blast is a minigame in Mario Tennis for the Game Boy Color. The player controls Mario, and the gameplay involves hitting the Boo in the middle of the court while rallying with Luigi. The points double for each consecutive shot at the Boo, and the counter resets to 1 when the player misses the Boo. The game ends when the rally ends. Luigi's shots do add up to the score. In this minigame, there are three levels. In level 1, the player must earn 30 points; in level 2, 60; and in level 3, the most the player can get.