Talk to the group of soldiers as you approach them, specifically the soldier who approaches the screen, to receive an Ether. Talk to the other guard that remains stationary to receive a Hi-Potion. Clasko will approach you once you get far enough down the road and will explain that you need to head to the left to get to the command center. He will also stop you if you try to continue down the Mushroom Rock Road.
O’aka XXIII can be found around the corner. Feel free to buy any items you might be short on and then continue down the pathway.
Mushroom Rock - Valley
Save at the Save Sphere before you continue and then talk to the guard standing at the end of the pathway for a free Hi-Potion. Stand on the panel and press ‘X’ to have it lift you up to the next level.
Follow the pathway along - there are no turnoff points so you cannot get lost. You will encounter a guard and a treasure chest along the way. The treasure chest contains 1,000 gil. Speaking to the guard will net you ten regular Potions. Don’t forget to grab the treasure chests along the way - one contains a Remedy and one contains a Hi-Potion. Ride the platform (by pressing ‘X’) once you get to the bottom.
Mushroom Rock - Precipice
Talk to the soldier standing right in front of you when you get off the lift to receive an X-Potion. Before you go straight to the next platform and before you go in the direction that the red arrow on the mini-map is pointing, you should take a left down the pathway shown on the mini-map. You will see a green square at the end of the pathway. It leads to another platform (as shown in the screenshot below). Take the platform down and you will find a treasure chest that contains a Serene Armlet.
Take the lift back up and continue down the pathway. As you continue down the trail you will come across yet another pathway that goes off to the left side. At the end of this pathway you will find Al Bhed Primer Vol. X. Now go back to the main path and take then elevator up to the next level.
Head around the corner and make sure to save your game at the Save Sphere. Go south from there to initiate another scene between Gatta and Luzzu. Speak to Luzzu once the scene has ended. Proceed up the lift once the scene has ended and continue along the pathway for another cutscene involving the machina.
Mushroom Rock - Ridge
Continue along the pathway and you will come across another Save Sphere. O’aka XXIII is located just behind it if you want to stock up on any additional items. Follow the pathway until you run into Gatta. Following that the team will have a conversation with Wen Kinoc.
There is a chest back near Yuna and Lulu that contains a Mega-Potion and another one even further back that contains a Serene Bracer. Make sure to grab both before you continue and then talk to the guard when you are ready.
Boss Battle: Sinspawn Gui
The strategy for this fight is fairly straightforward once you take into consideration each of Sinspawn Gui’s body parts and their behavior. There is the head, two arms and the body (the body is the actual boss and is labelled Sinspawn Gui when you initiate an attack).
The Head: The head will attack with Thunder primarily. You will sometimes see a message on the screen indicating that ‘The head is moving suspiciously’. This means that the head is about to attack with Venom, a very powerful attack. Attacking the head will prevent the head from casting Venom. You will need to use either Wakka or Lulu to reach the head and it is recommended that you hit it with an attack every time you get the warning message.
The Arms: Each of the arms will deflect attacks away from the body, Sinspawn Gui. You will want to destroy both arms prior to attacking the main body. The arms will eventually regenerate but there is nothing that you can do to prevent this.
The body (Sinspawn Gui): The body will alternate between two attacks, Punch and Demi. Both hit quite hard so be prepared to heal your team as they take damage by swapping in Yuna.
The strategy for this fight is fairly straightforward. Make sure to use Tidus’ Haste ability on all party members and use Lulu’s fire based magics as often as you can as Sinspawn Gui and its body parts are weak against fire magic.
Boss Battle: Sinspawn Gui (Round 2)
This version of the fight is much more simple as Maester Seymour has a number of powerful attacks that you can use. In addition to this, Sinspawn Gui’s different body parts have much less HP for you to deal with.
One Fire magic from Seymour to the head of Sinspawn Gui will take care of it. Then just hammer away at the body until you defeat it.
You will be back in control of Tidus when the game resumes on the beach of Mushroom Rock. Travel to the back of the beach near the cliffs to find Gatta and resume the story.
Use the Save Sphere and move to the left to continue the story along. Make sure to grab the treasure chest off to the left amid all of the talking cutscenes. The chest contains a Hi-Potion. Follow the pathway around to meet up with your team members again. You can speak with Shelinda as you pass to get a free healing. O’aka XXIII is off on the right hand side of the screen if you would like to buy any weapons. Make sure you buy some Softs from him before you continue (at least 10).