Instructions
Reach the end of mission 66 in Halloween Town until you reach the cut-scene that introduces the Leech Grave villain.
Press the "A" button to skip the cut-scene and begin fighting Leech Grave. Press the "Y" button on your Nintendo DS to select the leviathan keyblade weapon.
Press the "A" button to select the keyblade. Use your direction pad to move towards the Leech Grave.
Press the "B" button to jump up to avoid attacks from the Leech Grave's tentacle claws. Move forward to the claws when they aren't attacking you and press the "A" button. This will stun the Leech Grave.
Move forward while the Leech Grave is stunned and press the "A" button when you're near the gravestone body. Continue attacking this area until the Leech Grave is fully defeated.