For this trophy you'll have to earned an S Rank for the battle of the Nine-Tails' Attack, which is also in conjunction with the Minato vs Masked Man battle.
For this battle you'll partake in a 2-part Boss Battle against Masked Man and the controlled Nine-Tailed Fox. For the S Rank trophy you'll need to do exceptionally well in each individual match, as your efforts will be conjoined for an overall Rank at the end of it all. Here are some conditions that'll help you in your attempt to S Rank these battles:
LEGEND palette- Grants offensive Ninja Tools and increase stock and item upgrade with every level.
HERO palette- Grants healing/defensive Ninja Tools. With increase stock and item upgrade with every level.
-Maintaining good health
-Achieving bonus requirements
-Unlock Secret Factor
-Unlock Secret Action
-Achieve good enough RANK for BOTH halves of the battles
-Choosing your decision palette wisely, depending on the items and situations. (For later Boss/Pseudo battles)
-Using Bento and items prior to battle will INCREASE your chances of survival and a high Rank.
Bonus Requirement factors that are needed to rack up points for better ranks during normal battles are as follows:
-Ultimate Ninjutsu used
-Team Ultimate Ninjutsu used
-Canceled Chakra Dashes
-Com Difficulty Bonus
-Ninja Tools used
Bonus Requirement factors that are needed to rack up points for better ranks during Mob Battles are as follows:
-Attacks from behind (A button feature)
-Consecutive Burst actions
-Ninja Tools used
If done correctly you'll achieve an S Rank, as well as the A Rank trophy as well.
If, for any reason, you aren't able to initiate the Secret Actions no worry. You can retry ANY battles post-story in the Ninja World timeline for another try.