Final Fantasy X/Mushroom Rock
This is Mushroom Rock Road — quite a contrast from Mi'ihen Highroad, isn't it?
 Getting to the Command Centre
Again, it's important you talk to people — they'll give you stuff. After Seymour walks off and Wakka's faith has been shaken a bit, carry on along the road until you reach the junction. Clasko, a Chocobo Knight will direct you to the left-hand path, so you can't just go along straight through Mushroom Rock to Djose. If you go left, O'aka's there again. Make sure you buy some items off him too, if you haven't already been, since from now you'll start to encounter status effects more often and the curing items will become useful. You might also want to buy some equipment too; the Magic Bangle for Lulu and the Metal Shield for Tidus are good purchases, although you can afford to avoid them if you don't want to pay for it.
Carry on as you would along the road. You'll encounter some more new monsters here, as well as some returning ones from Mi'ihen Highroad (especially if you took the lower path):
- Red Elements are elements like the one's you've encountered before, so just use their opposing magic element. Red Elements are Fire-based, so use Blizzard on them.
- Raptors are lizards, so they're quick. As before, they can inflict silence and petrification on you, so watch out, the only difference this time is that they have slightly more health. Tidus can still kill them in one hit, though.
- Funguars are a new kind of enemy. The most important thing when you fight them is that you do NOT physically attack them. If you do, there's a good chance they'll use Pollen, which puts everyone to sleep. Not good. One Fire from Lulu is enough to kill them.
- Vouivres and Lamashtus are the same species of fiend — armoured dragons. The difference between them is that Lamashtus have slightly more health. One hit from Auron is enough to kill either of them. Vouivres will appear along the early parts of Mushroom Rock Road and more on Mi'ihen Highroad (the Oldroad, to be precise).
- Gandarewas are another new kind of monster. They're aerial, so they'll dodge most normal attacks except Wakka's ball, and Lulu's magic is somewhat ineffective against it. So, just hit it with Wakka and that's job done. They'll mostly only use Thunder.
- Garudas are exactly the same as the predetermined battles against them before — the only differences are that they have 4000 HP as opposed to under 2000 and they appear in random battles. Just use Dark Attack as necessary and it should never land a hit.
There's a long section up next (ignore the inaccessible save sphere). The chest contains 1000 Gil and the man next to it will give you 10 Potions. Another chest further along contains a Remedy. Another hidden behind a rocky pillar contains a Hi-Potion. Once you reach the end a rock column lift will take you up to the next area.
Talk to the guy in front of you for an X-Potion, and check the lift to the left a little further ahead to find a Serene Armlet for Kimahri. Further ahead there's a spiral path to the left which has Al Bhed Primer X at the end. Take the lift up to the next tier of this area, and talk with the people surrounding the lift for some more goodies. You'll have another few important scenes with Luzzu and Gatta if you head down to the south, then you can take the lift up to the Command Centre (save first).
You'll see some of the stuff they'll be using; cannons of some description, and a particularly impressive energy weapon that you'll see soon enough. Follow the red arrow as usual. Just before you go inside the command centre, use the Save Sphere. O'aka for some reason just got a huge bundle of equipment that's significantly better than anything you've got at the moment, but it's also expensive. The T.K.O. and Sentry are overpriced and not that useful, so the Avenger is the only thing you should really even consider buying, but at 6075 Gil it's not cheap, so it doesn't particularly matter. Once you're done, try and go inside.
After a few scenes with Gatta you can head inside and speak with Maester Kinoc, another Maester of Yevon. As the man says afterwards; the operation's going to be dangerous, so make sure you're prepared. Save again if you need to, and hunt down O'aka if you want to buy anything else. Before you talk to the man to proceed onwards, check the top of this area for a Serene Bracer for Auron and a Mega-Potion. Talk to the man to begin the operation. The sinspawn inside will break out of its cage…and land right next to you.
 Boss Battle: Sinspawn Gui
This looks quite menacing, doesn't it? There are four parts to Sinspawn Gui — the two arms, the main body and the head. Each arm has 800 HP, the head has 4000, and the main body has 12000. First things first, you'll be up against a variety of attacks. The head will use Thunder on one person for about 200-300 damage, and Venom after it "starts moving suspiciously" which does about 300-400 and inflicts poison on one person. The main body can use Demi which knocks 1/4 off your current HP, and a punch of sorts which does about 750.
Now, let's go on to beating it. First of all, switch Tidus in if you need to and have him cast Haste on Lulu (providing she's in the battle, otherwise switch someone in for her first). Wakka should switch in as well. Tidus' next turn should be casting Haste on Wakka. Wakka and Lulu should both keep attacking the head (which is out of range of normal attacks) until you've depleted its 4000 health. Once Tidus has done his part switch him out for Yuna — she'll need to keep Wakka, Lulu and herself healed up until the head is defeated. The head is more a pain than anything else — it makes the battle easier if you get rid of it (it won't regenerate either as the arms will).
Once the head is down, bring out Auron and Kimahri. If either have full Overdrives then use them (Kimahri should use Fire Breath if he has it, otherwise wait until the arms are destroyed then use Jump on the main body). Switch Tidus back in to cast Haste on both of them. If he gets a spare turn use Cheer, or if he has an Overdrive wait until there's no arms left and use it on the main body. The arms will regenerate — when this happens simply use Kimahri and Auron's piercing weapons to destroy them again. Bring out Yuna once more to keep everyone healed. If Kimahri does less than 300 damage to the main body and there are no arms out, switch him out for Lulu who will be able to deal damage faster (especially if she's in Haste). If you're bored feel free to bring out a summon to help deal damage. Ifrit's Hellfire by this point can do about 2000 damage to everything, including the arms. Unfortunately, in the waiting time until Ifrit gets his next turn the arms will probably regenerate. However, magic-based attacks bypass the arms, so you can use this to your advantage to hurt Sinspawn Gui whilst you destroy the arms.
After you beat it, you'll get a pretty, yet disturbing scene where everybody starts the attack. Returning to Yuna and her guardians, you'll see Seymour wrestling with a reinvigorated Sinspawn Gui.
 Boss Battle: Sinspawn Gui, Round 2
Exactly the same properties as before, except the main parts of Sinspawn Gui have much less health (apart from the arms which have the same), you can't switch anybody out, and you have Seymour in your party. Wow. Seymour:
- Has all the -ara elemental black magic spells (Fira, Blizzara, Thundara and Watera).
- Has Cura, the next version of Yuna's Cure.
- Can do similar damage to Tidus and Wakka with a physical hit.
- Has 1200 HP and 999 MP.
- Has an Overdrive, which, needless to say, is powerful.
But it's difficult to get his Overdrive meter cranked all the way up without accidentally killing Sinspawn Gui because one -ara spell to the head overkills it, since it only has 1000 HP this time. Same goes for the body, which has 6000. This is the only opportunity you get to see Seymour's Overdrive, but it's not really necessary. Either way, depleting 6000 health is really no challenge in this battle.
Finishing this battle will trigger the after-battle results screen, but don't get your hopes up — Seymour does not join your party as the 7th member and you can't do anything with his Sphere Levels. Luckily though if you used everyone they'll all get a large amount of AP and a few Sphere Levels along with it, accompanied by a lot of Gil and Level 1 Key Spheres. Sadly, the battle against Sin itself is far from the same; as you'll see.
There's another group of scenes for you to watch, and the carnage Sin brought will be shown once more. Check the chest underneath the ruins to the left for a Hi-Potion, and save. Head up the ramp and around to the left. You'll rejoin your party and be off to Djose.
You could easily train on Sinspawn Gui's arms since they keep on regenerating; just kill them and wait until they regenerate.
 Djose Highroad
You're not quite at Djose Temple yet, so there's a long road section you need to travel along. Make sure you run after the people on the road — one of them has a Soft Ring which is really useful for a new kind of enemy you'll encounter here. They're Basilisks, and they can petrify you, so the Soft Ring on Yuna grants her immunity to it. Kimahri can learn the Stone Breath overdrive from these guys,so don't forget to use lancet. The chest a little way along this road contains 2 Phoenix Downs, not too far ahead is Al Bhed Primer XI slightly hidden behind a stone pillar. Further along there's a Bright Bangle in an alcove. Carry on along the road to the entrance of Djose Temple.