This achievement can also be taken into account if you want to earn the S rank trophy as well.
For this achievement you'll have to earned an S or A 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 or A Rank achievement 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 factors that'll help you in your attempt to S or A Rank these battles:
-Maintaining good health
-Achieving bonus requirements
-Unlock Secret Factor
-Unlock Secret Action
-Achieve good enough RANK for BOTH halves of the battles
If done correctly you'll achieve an A Rank! If, for any reason, you aren't able to earn an A Rank no worry. You can retry ANY battles post-story in the Ninja World timeline for another try.