Clear as much as you can, give yourself about 12 moves left, then Starwhal!

OK, I played this probably 50 times. Finally, I had cleared the bottom gravel and knocked a few off the side ones. They turned light brown at 3 moves to go. I placed a star swapper and flame gem smack in the middle and BAM! Make sure you have at least 1 hypercube, 2 star swappers, and 2 flame gem powers in your arsenal.