I beat every Boss (including Vantitas in Terra, Aqua, and Ven) on the first try. Basically, every time I run into any Unversed, I beat the snot out of them until there are none left in the area, and that causes you to level up insanely fast and insanely early on in the game, so you don't have any problems with the big guys later. Even in the Final Episode, it only took me one try to beat You Know Who. Also, the Mirage Arena, the Meld function, and the Command Board definitely need to be taken advantage of, because they'll help you out tremendously.
~Ness