In Chapter 12, when you have the Left Tablet and are heading back to put it in with the other two, you will go over the drawbridge. When you get across the drawbridge there will be three purple monsters with big swords. Kill them and get the special box. Use the Unlabored Flawlessness or the Dabilahro to kill the monsters (stay a little bit back and use triangle to power to maximum).
When the monsters get close enough, let go. This should take about 3 hits on each monster. Then either go down the stairs or jump off the ledge to the save point below. Save your game, then head back up the stairs and kill the purple monsters. They will continue to put out yellow essence as long as you kill them.