Planet 51

As soon as the mission begins drive down to one of the lower corners, right near the drain. Begin pressing the “X button” which scans the falling rocks. Any that glow blue after being scanned are “Special Rocks” and even when not scanned the “Special Rocks” glow a pinkish orange. When you see a special rock, slightly wait until it’s close to your location and press the “Y button” If you wait a moment to grab them rather than racing up the hill, it will prevent you from missing any “Special Rocks” that fall right near the drain. Some say this is the hardest achievement in the whole game. But using the method I have listed, I was able to achieve this on my 2nd attempt.