For this achievement, you’ll have to finish at least 200 battles with an S Rank.
To do this, you must score enough points through the course of the battle – 1000 – taking into account a number of factors, including skills used, Head-to-Head victories, Kinship skills used, and time/turns taken.
Note that, despite displaying S Rank when attacking and killing weaker monsters in the overworld, these victories do not count towards the medal total. However, Quick Finish victories DO count as they take place within the main battle screen.
Once you’ve achieved S Rank in battle 200 times, the achievement will be yours!
Like many of these achievements, you can check your progress in your Ranger Records by pausing the game, navigating to “Ranger Files” (one tab to the right), and then selecting the Ranger Records dossier (the red one on the right). The Supreme Crest medal is the second medal on the third row.