After venturing through the dinosaur-filled Lost Underworld, you reach the location of the final Your Sanctuary.
About the Star Pendant
In Fire Spring, you can fight Major Psychic Psychos, who have a 1/128 chance of dropping a Star Pendant. This item protects against fire, freeze, flash, and paralysis, and provides the highest defense of all items that can be equipped on the body. If you want to try hunting for the Star Pendant, make sure to leave at least one space free in Jeff's inventory so he can successfully steal it from Major Psychic Psychos if he uses the Spy command. If Jeff's inventory is full, the Spy command won't tell you whether or not the Major Psychic Psycho is carrying anything.
Fire Spring Cave
Go through the door near the entrance, and go to the right side of the cave to find a gift box. It contains a Speed Capsule.
Leave that room, then go up the nearby rope. Go up the next rope, then go through the door. Go through the door on the right. Then go down the rope and open the gift box to get a Bag of Dragonite.
Now go back the way you came, until you have gone down the two ropes. Go east, and after you go past a small pool of lava, go north and through the door there.
Go east through this small cave and go through the door on the right.
Go to the southeast corner of this area to find a gift box that contains a Cherub's Band. Then go west again, and go up the rope next to the door. Go up the next rope, and go into the cave at the top to find a gift box that contains a Horn of Life.
Leave that cave and go down the rope, then go through the door on the left to find a Magic Butterfly room.
Then leave the Magic Butterfly room and go through the door to the right of the rope. Go through the door on the right in the small cave, then go up the rope. Go west and go up the rope there. Go north and go through the door. Then go west and go through the first door that you find. Climb the rope, then go east and climb down the rope to find a gift box that contains a Moon Beam Gun.
From there, go up the rope, go west, go down the rope, and go into the cave. Then go north, then west, and through the door on the left.
Go up the rope, then go east and go up the rope there. Give the Flame Pendant to whoever has the lowest max HP. Make sure to equip protection against Flash (Sea Pendant, Night Pendant, etc.) Then go east and talk to the sparkle to start the boss battle.
Don't use Multi bottle rockets. Carbon Dog is strong against Fire, so use Freeze against it. Carbon Dog can attack with PSI Fire Ω. Carbon Dog has 1672 HP. It transforms into a new monster after you defeat it.
Diamond Dog starts with Shield β, so do not use Multi bottle rockets or physical attacks until Jeff uses the Neutralizer to get rid of that shield. Even after the shield is gone, you might want to avoid using physical attacks in case Diamond Dog puts up a Shield β on its turn. Attack with PSI Freeze and PSI Rockin. Jeff can use the Hungry HP-Sucker to avoid reflecting damage if Diamond Dog puts up a Shield β.
Ness will soon be on his own for a while, so equip him with the Sea Pendant (or a Star Pendant if you got one from a Major Psychic Psycho), and give him the Franklin Badge. You can optionally leave the cave to buy Super Bombs for Ness. You can give him Bags of Dragonite if you have any. Don't give him the Casey Bat. Make sure that he has some HP and PP healing items as well.
If you believe that Ness is ready, go through the door that the Diamond Dog was guarding, and follow the path to reach Fire Spring.