Bug Fables: The Everlasting Sapling

This achievement, as the title suggests, is awarded after receiving the max rank in the game. The max rank is rank 27, in this case.

The "Hard Mode" medal obtained by speaking to Artis in the Explorers' Association after the opening sequence of the game and the "EXP Booster" medal obtained by make this go a bit faster with added difficulty.

You should reach this level normally just progressing through the game and fighting everything you come across or at least close enough to mop up some enemies at the end of the game. I recommend always having the "Hard Mode" medal on by default as it directly ties into many other achievements, however.