From the Super Mario Wiki
Giant Spiny Shell is an item found in a few Mario games. The Bowser Shell is a type of Giant Spiny Shell, as it is a very sharp and spiny Gargantua Koopa Shell.
Giant Spiny Shells act as items that the players can use in Mario Strikers series. It works similarly to the Giant Green Shell's, which are also items in the game. When fired, the shell will be fired towards the opposing players. However, the Giant Spiny Shell is much more durable than a normal Giant Green Shell, and will keep rolling around the stage, even if it hits a player. They continue to squish players until it finally wears out, and destroys itself, sometimes exploding suddenly, knocking out more players in a more larger radius. However, it lasts longer than the normal shells, so is much more of a hazard. The shells can turn against the team who used it, so that's one backfire for the user. The smaller counterparts are the normal Spiny Shells.