If you're looking for certain enemies in certain areas, try the following. Every level is separated into regions, and certain mobs only appear in certain zones. Simply find the enemies you need and kill off everything in the area, including the enemies you need to kill. Then, head to the portal leading into the next region, but don't go through it.

Instead, press select, quit your game (making sure to save), and then reboot your saved game thereafter. You'll have full energy, full SP, and be in the same region. What's more, all of the enemies you killed prior to the save/quit will be back. This is especially useful if you find an area that has an enemy or group of enemies that you need to kill mass quantities of.