Set the game to Legend difficulty and play the "Vorios" main mission. Reach Level 6 to defeat the Walking Glacier Boss at the start of the "Icecreek Sanctuary" main mission. Once done, pause the game and choose the "Quit Mission" option. Reload the "Icecreek Sanctuary" main mission and defeat the Walking Glacier repeatedly until you reach Level 18. You can then complete all side and main missions up to the "Azabrak Ruins" main mission. You will also be powerful enough to complete the "Gloom Vale" side mission. Keep replaying and completing the "Gloom Vale" side mission until you reach Level 29, then complete the game to unlock the "Deity" difficulty. Choose the "Deity" difficulty, then play the "Icecreek Sanctuary" main mission again and keep defeating the "Walking Glacier" until you reach Level 50.