There is a Moogle named Mimoza standing at the entrance to Oeilvert. Select the Mognet option and choose to take a letter from Mimoza addressed to Mooel. You can use the Mogshop to purchase a ton of great new items including:
Diamond Sword
Trident
Mythril Claws
Magic Racket
Healing Rod
Fairy Flute
Cypress Pile
Silver Fork
Pinwheel
Chimera Armlet
Egoist’s Armlet
Thunder Gloves
Diamond Gloves
Mantra Band
Dark Hat
Green Beret
Gold Helm
Cross Helm
Brigandine
Judo Uniform
Plate Mail
Gold Armor
As well as the standard set of consumable items. Make sure to purchase quite a few Green Berets and have some of your team members start learning the 
Clear Headed ability which can be used to prevent the Confusion status effect.
You can use a 
Soft on an enemy in this area (called an Epitaph) to instantly destroy them so make sure that you have a stockpile of Softs as well (shown in the screenshot below). Save your game and then proceed forward.
A short cutscene will initiate once Zidane reaches the entrance into Oeilvert. The large door will automatically open up allowing the team inside.
There is a treasure chest under the stairs at the back of the room that contains a 
Remedy as well as another chest at the top of the stairs that contains a Rising Sun item.
Directly west from this chest at the top of the stairs is a room with a blue orb on a pedestal. Touch the orb to activate it (switching its color from blue to red) and then grab the treasure chest behind it to obtain an 
Elixir. Go back into the previous room and take the stairs on the right side.
The first thing you will notice in this room is the Moogle at the back and Stiltzkin standing next to it. The Moogle is Mooel - give the letter from Mimoza to Mooel using the Mognet option to pick up another Kupo Nut (check out the Kupo Nut side quest section for more information).
Speak to Stiltzkin and give him 888 gil to receive a 
Hi-Potion, 
Emerald and Elixir. Lastly, there is a treasure chest on the opposite side of the room that contains some 
Gaia Gear armor.
Driving the Hard Bargain
You must purchase all of the available items from Stiltzkin each time he appears in order to obtain the Driving the Hard Bargain Trophy. This is a missable trophy as you cannot return to some sections of the game in order to buy items that you have missed.
Return to the previous room (the entrance to Oeilvert) and take a left this time. There are two treasure chests that you can obtain in this room - one contains a Diamond Sword and the other contains a Shield Armor.
A short cutscene involving a hologram of a planet will play as you walk around the yellow light in the center of the room. Take the stairs at the bottom of the screen downward.
There are two treasure chests in this room as well. One contains a Power Vest and the other contains 
Feather Boots. There are four yellow lights in this room that have to be touched in the correct order.
Start by touching the light on the left on the upper platform. Light up the one to the right of the first (close to the treasure chest that had the Feather Boots in it). Light up the two on the lower platform (right one first and then the left one). Take the stairs at the top left up and out of the room.
Follow the pathway along until you reach a large orb in the center of the room. Touch the large orb to make another hologram appear.
Continue left down the pathway after the hologram has stopped playing. Go through the locked doorway at the top left side of the screen for another cutscene.
The cutscene takes place in a room with a number of faces on the far wall. The faces tell the story of the rise and fall of a civilization.
Return to the room where Mooel and Stiltzkin were standing in and Zidane will notice that the center of the room looks different than before. You should take this opportunity to use a 
Tent and save your game by speaking with Mooel in preparation for an upcoming boss battle.
Equip all of your characters with the Clear Headed ability in order to prevent the Confusion status effect. The Green Beret hats have the Clear Headed ability so you can equip each character with it as needed.
You can also equip characters with the Bird Killer ability but this is not required. Lastly, equip your characters with any equipment that you have which reduces Wind elemental damage. This includes 
Lamia’s Tiara, Gauntlets and Shield Armor.
Take the exit on the far side of the room and ride the elevator down. Inspect the device on the wall to cause it to drop down and reveal the Gulug Stone. The boss battle begins as soon as you attempt to remove the stone.
Level: 38
HP: 20,002
Ark has a number of attacks that it uses during the course of the fight. The most devastating of these attacks is a move called Photon which will reduce one party member’s HP down to 1. You won’t be able to use healing spells so have a character ready to use healing items on any character hit by the attack.
Ark will use a move called Whirlwind which does not do a lot of damage but will cause the character to float (the Float status effect). Any floating character will take more damage when hit by a wind-based attack. Propeller will cause any character who does not have the Clear Headed ability equipped to be hit with the Confuse status effect which can devastate your party so proper preparation for this fight is required.
Just use regular attacks, Freya’s Jump ability, etc. until you bring Ark down and buy a stockpile of Hi-Potions if you are having trouble keeping your party members alive.
The items that you can steal are not really worth stealing so don’t waste your time.
Steal:
Elixir (common)
Power Vest (uncommon)
Holy Lance (rare)
Drops:
Pumice Piece
Ether
A video of the battle is included below:
Grab the Gulug Stone out of the device after the battle and the game will automatically return to the other team back at the Desert Palace.
