From the Super Mario Wiki
In the game, the object is to throw baseballs at generic ghosts from Luigi's Mansion. The player uses the Wii Remote and Nunchuk to aim at the ghosts and hit them with baseballs. The player can throw in all four cardinal directions. If the ghost matching the player's color is hit, the player gets more points. Also, if the player throws successfully consecutively, a consecutive bonus is awarded. Occasionally, a bigger ghost appears which takes many hits to destroy, getting smaller as it gets hit. All players can hit it, but only the person who defeats it gets points. Ghosts holding stars, or rarely Bob-ombs, occasionally appear. Whoever hits them gets the item. Stars make the player invulnerable, while bombs stun him or her for a couple seconds.