You can get this right in the tutorials. Place the cannon on the top left platform, hit "A" twice, no need to aim. Hold "X" to make the level go faster. As soon as the 1st splat jumps down, hit "A" to fire the cannon into the boulder, it will kill the splat. Hold "X" again. As soon as the 2nd splat jumps out press start/reset. Rinse & repeat. Took about 10 mins.