Instructions
Unlocking Special Mode
Insert "Mario Superstar Baseball" into your GameCube and turn it on. The opening cinematic will start to play. You can either watch it or press any button to skip it and head to the main menu.
Select "Challenge" from the main "Main" menu. "Challenge" is the second option down directly below "Exhibition Game" and above "Toy Field." "Challenge" mode consists of five games with the last game being played against Bowser.
Pick a team captain and begin "Challenge" mode. The easiest and only difficulty level available at first is the "Mushroom" difficulty level. It is worth noting that the four other team captains you do not select will be the captains of the teams that you will face in "Challenge" mode.
Beat "Challenge" mode on the "Mushroom" difficulty level to unlock the "Flower" difficulty level. Once again you must beat "Challenge" mode on the "Flower" difficulty setting to unlock the "Star" difficulty. Play and complete "Challenge" mode on "Star" to unlock the "Special" difficulty level.
Unlocking Bowser
Select "Challenge" mode from the main menu.
Select the "Special" difficulty setting. The "Special" difficulty level is the last option on the right next to the "Star" difficulty setting.
Beat "Challenge" mode using the "Special" difficulty level to unlock Bowser. Bowser is now available as a team captain for use in "Challenge" mode.
Unlocking Hammer Bro
Select "Challenge" mode from the "Main" menu.
Pick any difficulty setting you would like. The level of difficulty that you beat "Challenge" mode on has no bearing on unlocking Hammer Bro.
Select Bowser as your team captain. You must beat challenge mode with Bowser as your team captain to unlock Hammer Bro as a playable character. Once unlocked Hammer Bro can be used in the "Exhibition," "Mini-Games" and "Toy Field" modes.