Note: This game is also titled Digital Devil Saga: Avatar Tuner.
Successfully complete the game and save when prompted after the ending sequence and credits. Load the cleared game file to start with all previously learned combos and Mantras from the previous game session. You can also learn the Human Form skill and fight a bonus Boss.
When in the underground waterway close to the third save point, you will encounter a Pixie with about 300 HP. Knock it down to about one half of its HP (use physical immune to expel), then wait for it to call for reinforcements. A High Pixie (also about 300 HP) should appear. Kill the High Pixoe in one round, and the Pixie will summon another during the next round or so. Repeat this as many times as desired.