For this achievement you'll have to earned an S Rank for The Rumbling Coast battle with Choji vs The Gedo Statue.
For FACTORS on reaching high Rank for Boss/Pseudo battles please consult with this achievement solution:
For this battle you'll be playing as Super Expansion butterfly Choji against the Gedo Statue. This battle will be an average battle, but in titan form. All basic fighting methods work and swaying out of an attack for a counter will help, as it's a feature for this fight.
Once you've finished the battle with an S Rank you'll earn this 20G achievement.
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.