The keys on the second board and the firefly on the third depth are blocked by gems in colors nowhere else found on the board, making it impossible to remove them without using the color burst from the magic orb. Additionally, getting the keys in the first depth is difficult. Though, there are plenty of moves to get around.
Lead the water to the keys as soon as possible, while charging up the magic orb. Upon arriving depth 2, use the color burst on the red gems to remove the dirt blocks under them, as they cannot be removed otherwise. At depth 3, use the color burst again, this time on the yellow gems. The firefly should then be rescued easily.