The answers provided are all correct. But it should also be noted still that Smashing Ground does not target. If 2 monsters on the field share the value that would be the highest defense, the user selects which of those he would prefer to destroy. Even if the monster selected by the user would go face down somehow, the other monster would still be destroyed.
For instance, in your example you have a face up Aqua Madoor and Gravekeeper's Spy. Your opponent activates Smashing Ground and wants to destroy Spy. You activate Book of Moon to flip your Spy face down. Your Aqua Madoor is still Smashed.
Sorry if this is redundant to some, but still worth noting I feel.