The Final Your Sanctuary
There are many enemies here in the Fire Spring, so you should be prepared for a difficult time. The final boss in this area is also one of the hardest in the game. Keep in mind that enemies in Fire Spring are weak to PSI Freeze and use this to your advantage. You can also duck in and out of caves in order to lessen the number of enemies you are required to fight through. The Major Psychic Psycho enemy has a 1/128 chance to drop a Star Pendant which protects you against Freeze, Fire, Flash, and Paralysis. If you have the time and patience, try and nab one of those for your team.
The following route will take you through to every item in the cave. Read ahead and skip any items you don't want.
As you enter the Fire Cave, you'll see a cave room entrance directly ahead of you. Enter this room for a Speed Capsule. Backtrack and when you are back in the first, large room climb up the rope nearby. Enter the doorway and follow it through. Once back in the main area, climb down the rope to grab a Bag of Dragonite. Now you'll have to backtrack all the way to the beginning again. Once there head to the right until you reach another door.
Enter this door and exit the other side, back into the main area. Note the rope directly next to you. If you walk all the way to the right first, you will find a Cherub's Band which increases defense, luck, and protects against hypnosis. Go back to that rope and climb it. The door all the way to the left on the cliff face contains a butterfly. You can also climb up the next rope here and the enter the door to find a Horn of Life. To continue on, take the door on the right and climb both ropes on the other end.
Enter the door and head left into the first exit you see. Climb the rope outside and follow it around and and down another rope to find a Moon Beam Gun. Head back into that door again and continue on to the left and out a second exit. Climb both ropes and you'll encounter the final Your Sanctuary Boss, Carbon Dog. Before you fight, make sure at least one person in the party has a Night or Sea Pendant equipped.
If you forgot to bring the shield neutralizer and are low on PP, use Jeff to attack him directly, make Paula pray, Poo to use PSI magnet, and Ness to either use some PSI attacks or to heal everyone. With Poo using PSI magnet, you'll gain enough pp to use more PSI attacks. Eventually his PP will be gone and the fight will be easier.
Before moving on after the battle, it is a good idea to make sure Ness has the Sea Pendant and some of your good healing items, including Magic Truffles. Give him any Bags of Dragonite you might have as well.