Note: This game is also titled Digital Devil Saga: Avatar Tuner 2.
Successfully complete the game and save on the Normal difficulty setting, then play again from your cleared game file to keep your previously earned Mantra/Skill, Linkage, and Karma Ring. Note: You cannot keep those bonuses if your second attempt is under a different difficulty setting.
After starting your cleared game file, reach the battle where a Ghoul tells you that he is wearing a Power Ring. Defeat him to get the Berserker Ring. This item allows you to remain in Berserk mode during a battle without the Solar Data requirement.
Successfully complete the game on the Normal difficulty setting to unlock the Hard difficulty setting. Alternately, load a saved game file from Digital Devil Saga: Avatar Tuner.