This achievement is earned from your 4th Boss Battle against Orochimaru and receiving an S Rank. (Chapter 2: Their Reunion)
In order to earn an S Rank during ANY battle among the game you'll need to earn points based on governed factors among the game. Health, Time remaining, and Bonus factors are among the factors that will earn this achievement.
Bonus factors that will help you out are as follows:
Maintained High Health
Defeated enemy quickly
Attacked with a combo
Used Ultimate Jutsu
Used Team Ultimate Jutsu
Performed Substitution Jutsu
Performed Guard Break
Defeated a powerful enemy
Used Battle item
If you have the basic feel for the fighting style and can manage simple battles then you should do just fine. But if you fail to get an S Rank don't worry, you'll be able to re-do ANY battles once you've completed the initial Story Mode.
P.S. QTE's (Quick-Time Events) in the game will not only work towards another achievement, but they'll also replenish health if inputted correctly.
Upon completing the Boss Battle with an S Rank you'll earn this 20G achievement.