This page explains how to do the Rainbow Shell sidequest.
Read The Fated Hour for instructions to get a Ruby Armor, which is helpful in this side-quest.
Start the Quest
First, go to 1000 A.D. and fly to the small continent of Choras. It is in the southeast of the world map (press Select to view the world map). Go to West Cape (in the northwest area of Choras) and check behind the tombstone to get a Speed Tab.
Go to 600 A.D. and fly to the small continent of Choras. It is in the southeast of the world map (press Select to view the world map). You can optionally talk to Toma in the Choras Café to learn more about this side-quest. He is at the left side of the bar. He will give you a Toma's Pop. Then you can go to 1000 A.D., and go northwest to the West Cape. Check on the tombstone to use the Toma's Pop. You will learn about the cave of Giant's Claw. Be sure to check the back of the tombstone to get a Speed Tab.
In 600 A.D., fly to the island northwest of where West Cape is going to be. Check on the front of the mountain to go in. If you poured the Toma's Pop on Toma's tombstone, the mountain will be open and will be called Giant's Claw. If you did not, you can still check on the front of the mountain to go inside.
If you haven't gone to 65 million B.C. and traded for the Ruby Armor, go and get one of those. Or, you can use Ayla's Charm to get Ruby Armors from Gigasaurs that you will encounter in this area.
If you have Crono, include him in the party so you can use his Lightning magic. Equip two people with Lightning-absorbing armor. If you have a R'bow Helm, you can equip it on the third person in your party.
Follow the path and read Toma's note if you wish.
In the next room, you can sit in Azala's throne if you wish. Then go south to the next room.
Go west and approach the chest, and there will be a monster encounter. As mentioned previously, you can use Ayla's Charm to get a Ruby Armor from the Gigasaur if you need one (you should only need one). Use Lightning to weaken the Leapers, but be warned that they will sometimes attack with the lightning that they absorbed. The Gigasaur will immediately counterattack with lightning if hit with a lightning attack. After you defeat them, open the chest to get a Sight Cap.
Ignore the door to the left of the chest, because it leads to a dead end. Instead, go east and down the stairs, then continue going east. Don't use lightning on the Lizardactyls, because it will heal them. When they are gone, go northeast through the door.
Step on the upper switch to create a save point, and use it to save if you wish. You will soon be unable to use it. If you didn't open the chest beyond the skull millions of years ago, it is still there. Just check on the skull to open it, then go through it and open the chest if you didn't before. Then, near the save point, step on the left switch. Don't step on the right switch or there will be a monster encounter. Fall into the hole on either side to continue.
Don't touch the switch on the right. Step on the switch on the left. Go through the skull and check on the sparkle to get a Power Tab. Go south to the next area.
This room contains a lot of treasure. Go west to the ladder, and go down it. Go east and check on the sparkle to get a Power Tab. Go west and open the chest to get a Full Ether. Go back up the ladder, and go west. You can avoid some monster encounters by staying near the outer wall. Before you go through the door to the south, go north and open the chest (partly obscured by the wall on the right). It contains a Blue Rock, which can be equipped by Magus, Robo, or Lucca to enable their OmegaFlare Triple Tech. Go south through the door.
Follow the path, and you will reach two ladders. Go down the southern ladder (the northern one leads to a broken ladder that would make you have to repeat the dungeon so far). Go as far west as you can go, and check the left wall to open a partly concealed chest. It contains a ZodiacCape. Go east to the small slope and go south down it. Go east and open the chest to get a Lapis. Then go down the nearby ladder and go west to the door. Go through to the next area.
You can avoid a monster encounter by staying near the west wall as you go north. You can optionally approach the monsters from behind to make them run away. Go through the open skull and you will see a sparkle on the other side, which you will be able to reach later. For now, go up the stairs.
Step on the left switch to get rid of the monsters here, and go through the southwest door. Open the chest in this room to get a FrenzyBand. Then go back to the previous room.
There will be an unavoidable monster encounter. Afterward, go east and step on the switch to get rid of the enemies, then go through the southeast door.
You can optionally battle the Rubbles here for a lot of experience, but they may run away before you can defeat them. Go through the northeast door.
To continue, open the chest on the right and you will fall below.
Exit the cell and go west and up the stairs. Check on the sparkle to get a Power Tab. Then go back to the previous room.
Go east past the cell and go down the stairs. There is a save point here, so use a Shelter and save. Then check on the switch to the right of the bars to open the cell, and go north through the door in the back.
Equip everyone with armor that absorbs/protects against Fire, such as the Red Mail, Red Vest, and Ruby Armor.
Go north and try to walk past Rust Tyrano to start the battle. Rust Tyrano doesn't "remove def, store pwr" like his previous version, so just keep attacking with your best magical/elemental Single, Dual, and Triple techs. Whenever Rust Tyrano chews on someone, heal them right away.
Giant's Claw, Part 2
Go north to find the Rainbow Shell. Check on it and there will be a cutscene. Go south and there will be another cutscene.
You may wish to heal up in the beds in the southwest of the castle before you continue. Then put Marle in your party, and go outside and use Epoch to go to 1000 A.D. Enter Guardia Castle.
If you haven't entered Guardia Castle the Marle recently, the Chancellor will have some dialogue when you enter the castle with Marle now.
Go east from the entrance, then go into the stairwell on the left (the one with two soldiers in front of it), and go to the right to go up the stairs. On the way up the stairs, open the chest to get a HyperEther (if you didn't already). There is also a suspicious locked chest on the way.
At the top of the stairs, you reach the trial room. Talk to the guards and there will be a cutscene. Afterward, go back down the stairs, and when you are back at the landing guarded by two soldiers, go east to the other stairs and go down.
Go east toward the snakes to battle them.
After the cutscene, defeat the enemies that attack. Open the three chests that you can reach (one near the barrel, two to the left of the axes), to get a Lapis, an Elixir, and a HyperEther. Go north through the archway. You can avoid battling the Naga-ette if you can get past it without touching it. Go north to the next area.
You can walk slowly north this hall to avoid some monster encounters. Then go north, open the three chests that you can reach (one is south of the axes), to get an Elixir, a HyperEther, and a Lapis. Make sure Marle is in your party, and go north to the Rainbow Shell.
After the cutscene, equip all party members with gear that protects against Chaos, such as Nova Armor, Vigil Hat, and the Amulet. Then go south from the Rainbow Shell (run to avoid the Gnashers), and go south past where the Naga-ette was (or is) bouncing around. Go south, then west, then up the stairs. Go to the stairwell on the left and go up the stairs. At the top floor, talk to the guards who are blocking the doorway, then watch the cutscene.
Don't use Lightning, because it will heal Yakra. Heal whenever Yakra uses needle attacks.
After the battle, watch the cutscene.
Before you go to Melchior in the basement, you can optionally go to the southeast of the castle, go up the stairs and enter the trial room, and check on the sparkle to get the Yakra Key. Then go down the stairs and you can check on the locked chest to free the real Chancellor (not doing so will just change a game ending slightly).
Then go downstairs until you are at the landing with the two soldiers, and go east to the stairs there. Go down, and make your way to the back of the area. You can talk to Melchior to get a choice of which items you want him to make.
Here is some information about the Prism Dress vs. the Prism Helms:
+ decrease damage from elemental spells by 1/3
+ increases defense by 99
- only get one
- can only be equipped by Lucca, Marle, and Ayla
- increases defense only by 40
+ get three of them
+ also increases Magic defense by 9
+ locks status
+ can be worn by anyone
You will have a chance to get a Prism Dress by using Charm on an enemy later in the game.
Also, if you did the Sun Stone sidequest, talk to Melchior again and he will make Crono's best sword (Rainbow) and the PrismSpecs, which maximizes the wearer's attack power.