Crossbone Isle dungeon:
Defeat the final Boss and save your game after viewing the credits. Reload the cleared saved game file, then sail south from Tonfon. You will eventually find a “C” shaped peninsula at the edge of the world. Enter the whirlpool at that location then sail east from the exit. You will find an island with a cave that leads to the entrance of Crossbone Isle.
Defeat the final Boss and save your game after viewing the credits. Reload the cleared saved game file, then sail north of Haran island until you find some rocks shaped like a hand appearing from the water. Sail against the middle rock, then sail left into the purple fog until you reach the Ghost Ship. The Ghost Ship is where you can find the summon Azul, guarded by the Star Magician.
When at Apollo Sanctum and activating the device, after the cinematic where the men fall from the airship, run around getting into battles. The enemies have around 500 HP each and inflict very little damage. If Matthew has the Sol Blade equipped, he may be able to take down enemies with one hit. You can easily gain over 21,000 to 45,000 experience points per battle, along with a Water of Life.
At the end of the game, before you battle the Chaos Chimera, walk around the Apollo Sanctum to get into random battles with the Tua Warriors. It is recommended that you have one of your party members, that is not in your “battle party” equip the Lure Cap. They will almost always drop Water of Lifes. You can sell them for 2250 coins. Thirty of them equals up to 67,500 coins. 240 of them would equal up to 540,000 coins.
Burning Island Cave door:
First, get the Sol Blade from the Apollo Sanctum and complete the game. Save the game when prompted and load the cleared saved game file. Starting at Tonfon, sail back to the Burning Island Cave and go to the bottom of the cave. Find the previously locked door and select it. After some dialog, open your inventory and use the Sol Blade by selecting it. The room beyond is where the Summon Daedalus and the Ogre Titans Bosses are located.
Otka Island maze:
First, get the Sol Blade from the Apollo Sanctum and complete the game. Save the game when prompted and load the cleared saved game file. Starting at Tonfon, sail west until you come across a small island surrounded by rocks. Wait until a whirlpool appears and sail into it to reach the other side of the rocks with the island. Land on the island, go inside, and select the door. After some dialog, open your inventory and use the Sol Blade by selecting it. The maze is where the Summon Catastrophe and Ancient Devil Boss are located.