Instructions
Defeat the Trickmaster at the end of the Wonderland level using a combination of physical and magical attacks to seal the Wonderland keyhole and receive the first Navi-G piece. This Navi-G piece unlocks Olympus Coliseum and Deep Jungle.
Seal the Deep Jungle keyhole by defeating the bosses Clayton and the Stealth Sneak at the end of the Deep Jungle level. It is recommended that you complete the storyline in Olympus Coliseum before traveling to Deep Jungle so you are at a high enough level to easily defeat these bosses. You will receive the next Navi-G piece, which unlocks the next set of worlds. You must travel back to Traverse Town, however, before you can reach the new worlds.
Defeat the Opposite Armor boss after returning to Traverse Town by targeting its limbs first, then attacking the body. You will seal the Traverse Town keyhole after the battle and receive the third Navi-G Gummi.
Seal the keyholes in Agrabah, Atlantica, Monstro, 100 Acre Wood and Halloween Town by playing through the storyline and defeating the bosses in each world. You will be strong enough after these worlds to safely travel to Neverland. Defeat Captain Hook in Neverland to seal the Keyhole and receive the last Navi-G piece, which opens the path to Hallow Bastion, the final level.
Play through the storyline in Hallow Bastion and defeat Riku at the end of the level. You will be thrown out of Hallow Bastion after a cutscene and will be unable to return without taking a different route.
Return to Traverse Town and talk to Cid in the First District. He will tell you that you need the special Navi-Gummi piece to travel back to Hallow Bastion. Enter the Secret Waterway from the nearby alley. Talk to Kairi inside the Secret Waterway and then inspect the mural to find the hidden Navi-Gummi. You can now access every level in the game.