you need to get normal ending on both Gunvolt and Copen on the same save file, and then when both of them have cleared story on the same save file play the final stage again as Gunvolt or Copen...you will get the true ending and fight Gunvolt or Copen because they are each other's true final boss...whoever is the enemy, onces defeated will be revived by Mythl Lola and Joule granting more power