When you get to Crossbone Island, head to the southern part of it. You'll see an indentation in the mountains when you get to the southern part of the island, so head north to it to find the entrance to Crossbone Island. When the world map disappears, go to the west to find a vine that you have to use growth on. Repeat this process two times until you're at the top of the cliff. There will be three crevices to fall down here. The crevice on the left will take you to the north of those tree stumps and next to a shrub that you should whirlwind to expose the entrance to Crossbone Island Cave. The middle crevice leads to a chest with 1 coin, and the right crevice leads to a chest with a mint in it. When you go down the crevice on the left and whirlwind the door, enter it to enter the cave. Head north along the hallway until you get to a large staircase to go down. Do so and arrive at the first floor of Crossbone Island Cave.
On each floor in Crossbone Island there are several treasure chests. In this guide I will mention how to beat the boss in the first paragraph, how to get the treasure chests in the second paragraph, and in the third paragraph I will mention how to actually solve the room and get to the next one.
One thing to note about Crossbone Island is that there aren't random battle encounters, which is quite nice. Instead, there are "Door Enemies" that you'll have to fight before you can enter in a door.
Also, whenever you come out of a door in Crossbone Island make sure that you push the stump down so that you'll have easy access past rooms you've already gone through if you need to head out of the cave and then back in for any reason.
The first of these Door Enemies is a Hobgoblin and a Virago. These guys aren't all too tough, because you'll be free to cast all of your summons right off the bat (remember, there aren't any random battle encounters in Crossbone Island.) 3 Level-4 Summons should wipe these guys out of the cave, giving you a Lucky Medal and access to the first floor.
Location: Southwest corner of the room.
Item: 111 Coins.
Directions: Hop onto the ledge right below the stump and push it to the left.
Head counterclockwise across the gaps to get to the treasure chest.
Notes Before Obtaining the Next One: Head out of the room.
Location: Southeast corner of the room.
Directions: Hop onto the ledge below the stump and move it to the left. Jump
onto the platform with the stump and head right. The chest is directly east of
Notes.....: Head out of the room.
Location: Northwest corner of the room.
Item: Hard Nut
Directions: Hop onto the ledge below the stump and move it to the right. From
the ledge head to the left and move the stump to the north and move the log
there to the right. Head north along the stepping stone path on the left.
Notes: Head off of the platform but you need not go out of the room.
Location: North-center part of the room, not on a stepping stone.
Direction: Head counterclockwise all of the way around the outer ledge.
To get out of the room you'll need to make sure that you haven't moved any stumps yet (head out of the room if you have.) Hop onto the small stepping stone below the only log visible. Move the log to the right and head northwest to the next stump on a ledge. Move this log to the left and start making your way east until you can't go east any further. Once you're there, head north until you reach the northeast corner. Start heading west until you come to a ledge with a stump to the left. Move this stump east one square, then start heading back from whence you came (13?) Make your way to the stump in the middle that you haven't moved yet. Move this to the right and jump onto it. Now you'll want to make your way to the log you moved earlier northwest of the log you just moved. Move this log to the east, hop on to the platform and make your way to the long outer ledge. Head all the way down this ledge and out the door. When you exit the room there will be a treasure chest here that's actually a Mimic. You probably will want to fight him at this point for his experience points.