This is a side quest relating to the Turtlez.
Throughout the main quest you will pick up a side quest to collect the normin dolls.
Port Cadnix: The black Normin Doll is located on a wood box next to the inn.
Faldies Ruins: In B1F, the red Normin Doll is located at a dead-end next to a treasure chest.
Haria: The blue Normin Doll is located in the centre of the town.
Stonebury: The green Normin Doll is in the raspberry field at the west side.
Once all the dolls are collected Eizen will get a letter with a challenge. Head to Titania where you will battle Phoenix. He is Lvl 100 so this will be a difficult battle and he is able to revive himself upon death. I have seen him do it twice but the amount of health he gets back drops each time IF he uses that mystic art to revive.
Head back to the Northern dock for another skit and then head to the Empyrean throne and speak with the White Turtlez outside. After the skit, head to West Meirchio and speak to the White Turtlez on the cliff.
At this point if you havent already make sure you have collected 38 Katz and have unlocked Katz Corner. Talk to all the Katz there except the skit and movie Katz. Head back to the Emyrean throne and there will be a Katz at the North East side that you will need to talk to.
Go back to Katz Corner and Battle the Dark Turtlez at the North West side. Once the battle is over and the cutscene plays the achievement will pop.