First you will need to make sure you have Maruto unlocked and load into the Abyss of Foscari map (Unlocked by destroying the seal on the Lake of Foscari map). The "Seal" is located in the bottom left of map and will be marked with a question mark if you have the Milky Way Map. Go to the seal and evolve your Eskizzibur (do this by having a max level Eskizzibur and Armour at the same time. Now stand near the seal to break it and unlock the achievement.