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.