You earn this upon beating the dragon in the middle of Hexen Isle, the post-game area that you access via a warp in front of the final boss after loading a cleared save. The dragon is available to fight once you defeat all 8 bosses in the branches of the bonus dungeon. It is incredibly difficult as you would expect, and you'll need to go into this fight willing to use every item you have, especially elixirs.
For those trying to do this without having done much fusion, your best hope is to armatize with Mikleo and stay as far away as possible, letting the AI tank the hits. Holding down while you attack (while armatized with Mikleo) will maintain a good distance as long as you position yourself to an edge far enough from the boss. Use items and BG to keep the AI healed. You may want to use panacea bottles to prevent status ailments. Elixirs will also give status immunity. If the AI characters both die, prioritize reviving the seraph since seraphs can instatntly revive Rose (or Sorey) by armatizing. If you have a lot of melon gels I would use them on whichever human you're not controlling (probably Rose), and it's not a bad thing if the AI doesn't stay armatized because that would allow it to strike the boss' impact weakpoint more easily, generating more power combos.