Well see.
You randonly get Final Form, when you use a drie form.
so if you want to get it faster just go to The Olypus Coloseum and enter the Cerebres Cup and Drive into a form. then if you dont get Final form(Gray and White colors) thn just press start abd retry it and repeat that until you get it.

You gain the Black form-Anti form if you Drive alot in a Row without stopping. So dont drive alot. but you can if you want but i have trick to level up the forms easy, so i dont like getting it.