This mission is confusing, if you have the Japanese version and are a fully-English speaker. It's really simple once you understand it.
You must destroy 3 Guardians and collect the 'shining shards' that they drop. You defeat only three to get the mission gauge up to green.
To complete the mission fully, simply turn around and defeat the fourth Guardian and collect the final shining shard.
File:Guardian.png
Guardian Heartless