I had the easiest time doing it with Team Dark, second easiest with Team Sonic, and surprisingly, the hardest time with Team Rose (they stink anyways, except for Cheese).
For all teams (this works especially well with Team Dark), follow the boss to the circle area where it stops for a bit (get the Flying formation level up). Just keep going around the boss collecting rings and defeating the enemies there. Follow it to the second area (again get the Flying level up). You should have a Team Blast by now (if not get the rings there and kill the enemies), so activate it and start shooting the boss with Flying formation (this is why Team Dark is best with this strategy, becasue their Team Blast freezes everything). Just keep shooting it, you should kill it if you did things right, and you should get A Rank.
Oh and don't bother with the speed formation level ups, they are near impossible to get and speed does nothing in the battle except chase the boss on the straight-ways.
