Edge of Eternity

The way this achievement is believed to work is that you need to hit an enemy with all 8 elements - thunder, wind, earth, holy, water, fire, dark, and ice - to fill in their entire weakness board. Dark and Holy elemental attacks are hard to come by, though Selene will get spells for these later in the game (Gravity Pull for dark and Falling Stars for holy), and you can craft "shard" consumables to use in battle that do this type of damage as well. It's possible that after doing all attacks you'll also need to scan the enemy in battle by pressing  on your turn to access the status screen of everyone in the fight. Use / to cycle through your party members and the enemies. Once you have scanned 40 different enemies, the achievement should unlock. However, it is currently glitched and does not progress.