Magus Sisters (Cindy, Sandy and Mindy)
First, you must capture 1 of each monster in the Calm Lands and Mt. Gagazet. Return to the monster trainer in the Calm Lands to receive a small prize. Once you get all 21 creatures, go to the southeast corner of the Calm Lands on a chocobo. You'll see a feather on a ledge. Use it to get your chocobo to jump to a hidden ledge. Then, get off the chocobo and get to the Remiem Temple. Yuna must now face off against an Aeon trainer against every Aeon she has. You can heal between fights, so it shouldn't be too hard. Eventually, you win two items that will open the nearby door to the Fayth of the Magus Sisters. On the way out, you can challenge the Aeon trainer one last time for a special item. She uses the Sisters in the last fight, so use Anima for the last duel.