[KH2.5] Gummi Missions?

Okay. So. I beat LW and all the Data Org battles on Standard Mode. I keep hearing and seeing about "Clearing Gummi Missions" to get the Birth By Sleep secret ending for KH 2.5. However, no one seems to have a clue what this means. Lots of people say you MUST 100% everything in the missions, which seems a bit much to ask a Standard Mode player to do. Asking a Crit mode player, yeah, I can understand, but this?

I thought you'd just have to get C rank or above because that clears the mission and opens up the next mission rank for you.

I'm having so much trouble, however. I keep hearing of a donut ship, but I don't have any of the needed gummies for it. II'm currently trying Asteroid Sweep, and am stuck on Mission 3, I can't get past E rank because of the stupidly high score.

What in the world do I do?

Also, huge, HUGE thank you to everyone who helped me out with the data battles. The scariest moments were when Xigbar was on a tiny sliver of health and I had to go into Valor Form to try to hit him. . .and promptly went Anti form. That was tense :P