Strategy Guide: Kingdom Hearts: Strategy Guide Part 2 (Page 2)
Traverse Town
Your merry crew retreats to Traverse Town, where you discuss how to deal with the Heartless. You need another Navi Gummi to get back to Hollow Bastion; talk to Cid and he shows you where it is.
Go down to the underground part of the Second District-you can access it either through the alleyway behind the hotel or via Merlin's house-and approach the sun mural located in the corner. You then find the needed Navi and receive a new keychain weapon from Kairi.
If you talk to Cid again, he'll install the Navi into your ship. Prepare your party for the battle ahead and pilot the ship into the new warp hole to revisit Hollow Bastion.
Hollow Bastion Revisited



Make your way to the castle as you did before. Your ultimate goal is the Castle Chapel (the place where you fought Maleficent earlier).
Talk to all the princesses in the Chapel and then go through the dark passageway by the save point once again. Pass through the corridor and then go up to the Key to All Worlds in the next room to pay a visit to the Dark Depths.
You fight the Behemoth, who's a lot more cute 'n cuddly here than in his Final Fantasy appearances. He's pretty slow, but all of his attacks take off a huge chunk of Sora's HP, so watch your HP gauge and try not to stay in one place too long.
The Behemoth's only vulnerable spot is the spike on top of his head. It's most easily accessed by climbing up the boss's rear end and jumping onto his back, giving you a pretty clear shot at the horn. You'll get knocked off now and again by his lightning attacks, but it's not too hard to climb back on again.