In Aqua's story in Deep Space, you will encounter Gantu several times. The last one happens when you find Experiment 626 and Dr. Jumba. You must battle him. Stay close. Gantu shoots his gun most of the time which will not hit you if you are very close, landing combos. When he dashes, follow him and repeat the process.
In each person's story, you will have to option to visit the Mirage Arena. Along with Rumble Racing and Battle Arena, you can play on Command Boards. You will unlock more Command Boards as you progress in your story. In the end you will have six Command Boards unlocked. You will have to play and win all six Command Boards to unlock the seventh Command Board, Secret Board. If you want to reach Arena level 30, you must play and win all seven boards.
Defeat Vanita's Sentiment to unlock the Mysterious Figure in the Land Of Departure.
In Aqua's storyline, while in Ghost mode, air dash into one of Tinkerbell's balls of light. You will remain invisible, even after Ghost mode ends. You will reappear once you attack.
These Bosses are only available to fight with after you have cleared the Final Story. They are accessible during Terra, Ventus and Aqua's Story.