Pretty easy but if you are having trouble here's some quick guidelines:
- Get a good ammount of stars, not only for the secret factor, but because you are healed. If you have 100% HP when you finish the fight, you'll most likely win a S rank.
- In the second phase, use Sakura to destroy the boulder Kakashi's stands to charge up Chidori.
- Be sure to stand on a land patch when Kakashi uses his Chidori on the water, but be careful! Avoid the Water Dragon that's most likely heading your way from the sky.