Story related, Cannot be missed
This achievement is called "Release the Kraken." You get it after defeating the Kraken in The Cave of Tides. He is weak to lightning so hopefully you saved your Zeus' Wrath. I had six in my inventory at this point. I used Luneth as a Black Mage and in this level you get your first Level 4 Black Magic spell: Blizzaga. USE IT! I had Arc as a Knight and he basically healed and protected everyone. Refia was a Scholar and I had her use the Zeus' Wrath. She landed about 1100 HP for each one she used. Ingus was a Red Mage and used either Cure or Cura after every turn. Kraken went down in about 5 rounds using this method.
Any other questions, let me know in the comments.