This is the third elemental rock, but thankfully it's not as annoying or as long as either Air's or Aqua. You won't have to worry about the last elemental rock either for a while yet.
Gaia Rock Exterior
You probably guessed it; Mt. Mikage is Gaia Rock. Rejoice in the fact that Gaia Rock is not at all like the other rocks. It's got a boss which can't be killed, for starters, and an endless maze. So in that sense, it's actually much worse. But it's easily doable, so don't worry.
Approach the rock, but don't go inside. The only way you can go will end in tears if you attempt it now, so instead go to the left-hand side and climb the wall. Shove the pillar off, then climb around and move it onto its metal plate. Go all the way to the right, climb down the other ladder, then go back across to the left wall, and climb up. You can use the pillar to hop across and use the next wall to climb up to the next area.
This bit gets quite confusing. Go up the right-hand wall, and scale it as far as you can (you will need to use the partially-swinging thick green vines). Up here, use the next vine to climb down and grab a Nut from the chest if you want to. Use the groove here to slide down (you'll be at the top of where the left-hand climbable wall would have taken you). Climb up the wall above you, then head left. The whirlwind statue here will give you a hint on what you need to do with the vines - using Whirlwind will blow them around so you can use them to swing across gaps.
Go down the wall (not all the way) and try this out to cross to the other side of the vine. Move the pillar one space to the left, then go back across the vine and down. Climb up, push the pillar onto the metal plate, and continue scaling up the cliff. Exit the area to the left.
The next area is deceptive too; the vines will make you think there are tons of different paths. You'll actually find there is only one, with one or two obvious dead-ends. You'll eventually reach a wall you can scale to the next area.
This is where Gaia Rock gets even more annoying. The wall has many different ways of climbing it, but going certain ways will cause moai heads to emerge from the wall and shove you off, making you start again from the bottom of this area. Keep to the right for the first section (once the statues at the bottom go out of view). Hug the right-hand side again for the next part. When you get to another junction, go left, and climb STRAIGHT upwards. If you're a little bit too far to the right you'll run into a moai head that'll knock you off. After a short climb you'll clamber up to flat ground again. You're at the summit now, so just follow the path weaving between the rocks and leave this area to the right.
Don't move the pillar here - go to it and hop across. You can grab an Apple from the chest over here. Now go back, move the pillar all the way to the bottom of its groove, then jump across it. Follow the long, winding path until you reach the very top. The altar here appears to be the only thing here, but you'll recognise the Reveal symbol. Use Reveal and grab the weird Dancing Idol from the altar. Now, Retreat back to the base of Gaia Rock. Now you can go inside.
Gaia Rock Interior
Before, straight ahead is the only way you could go. That leaded to an endless maze and, past that, an undefeatable boss that recovered all its HP at the end of every turn.
But now, you can use the Dancing Idol on the familiar altar here, and the two sinister statues on either side will move out the way, allowing you past. Take the right-hand door.
Follow the long path here until you reach a patch of weeds. Use Cyclone here and a load of floating grassy platforms will appear, which will allow you to take the other door at the entrance of Gaia Rock (make sure you use Cyclone on all of the weeds). Now, either follow the path all the way back to the entrance or use Retreat and re-enter Gaia Rock. Once you're there, take the left-hand door. At the end of the path, you can use the floating grass platforms to get across to the door at the top of the room.
At the end of the path, you'll see a large room. Ignore the steps on the right-hand side for the mean time, and go left. Follow the path until you reach the next room. Use Cyclone on the weeds in here to uncover a Mad Plant. This relinquishes a Potion when you beat it. Ignore the long climbing wall to the right here, and go down the steps and through the door at the bottom.
The weeds around the altar don't hide anything, so go ahead and use the Dancing Idol. The statue will move, but let light through to the hole. This light ray will be reflected by a purple orb in the Serpent's lair, weakening it slightly. Now, if you were to go and fight it, you'd still lose. You need to get the other three beams of light on the Serpent.
Head back to the room with the long wall, and start shimmying across it. Go along the bottom first, then top, then hug the bottom of the climbable section around to the platform. Use Whirlwind and swing across using the vine, then decend the steps and go through the door. Don't use the groove in here, instead use Cyclone on any of the weeds to drop the altar down a floor. Now you can use the groove.
You could have got two other beams of light down here before, but it's quicker to get the altar down first and get the other three light rays all at once. Use the Dancing Idol on the two altars and Cyclone on the weeds. With all four beams of light blasting the Serpent, you've now knocked his regeneration rate down from a ridiculous amount to 30HP a turn. That's right - even being weakened this much it still regenerates.
Retreat to the entrance of Gaia Rock again, and this time you can take the middle path. You have to solve a relatively simple Move puzzle along the way, and once you're past you can head through the door.
Now it gets tricky. You're in the endless maze section. Don't even think about moving until you've switched one Venus Djinni with one Mars Djinni on Felix and Jenna. Set Growth to a Psynergy Shortcut next. Ordinarily it would take you ages and ages to navigate this maze, but Growth makes the plant in the middle tell you exactly where to go.
The way to go is:
Left, Up, fight Mimic for Game Ticket, Up, Right, Up, Right, Right, Down, get Rusty Mace from the chest, Right, Right, Up, Right, Right, Up (see how hard this would have been without Growth?).
You'll trigger a scene, where Susa's attacking the Serpent. Nothing works, and then he gets blown off his feet by a fireball. Save, put all Djinni back to their respective characters, and Standby 4 Venus, Jupiter and Mercury Djinni, and 3 Mars. The best configuration is Flower, Cannon, Spark, Blitz, and Shade were the only Djinni set (these are the most useful Djinni).
Now approach the Serpent and interact with it to start the battle.
Boss Battle: Serpent
If you didn't weaken it with all the light beams, watch with horror as it recovers all its health after your initial summon rush. Oh dear. But thankfully, you did use all four light beams, so it only recovers 30HP a turn (this will only slow you down a bit when you're resorting to Psynergies and attacks).
So, the first turn goes Judgement, unleash Cannon, Thor, Boreas, and the next turn should go Ragnarok, Meteor, unleash Blitz, unleash Shade. You may be in a bit of a pickle by this point, especially since Toxic Breath can inflict Poison and a fair amount of damage, and the fact that the Serpent attacks twice in one turn.
Unless you need to do any drastic healing, the next turn should go: unleash Flower, Serpent Fume, Shine Plasma, Cure Poison (assuming there is somebody poisoned, otherwise use Diamond Dust or unleash an attack Djinni). If anyone is downed, use a Water of Life (these are best put on either Piers or Felix) or Jenna's Spark. Turn four should go unleash Steel, unleash Spark (if someone's down, otherwise Serpent Fume), Shine Plasma, unleash Chill. The Serpent shouldn't survive this.
You'll go through a scene with Susa, then you can touch the tablet. This is Sand, the Gaia Rock elemental Psynergy that Felix learns.
"Wielder of Earth's might...lay your hands upon this stone. I shall grant thee to power to melt into the earth, the power of Sand!"
And with that, you've learned Sand. Before you leave, you can use some remarkable foresight and get an item that ordinarily you wouldn't know of until you'd gone all the way back to Izumo and talked with Lady Uzume, Susa, and Kushinada. Use Sand on the left-hand side of the serpent, and you can get underneath its body. Check the tip of the stream of blue blood, and you'll get the Cloud Brand! Equip this on Piers, then use Sand again and leave the room. Use Retreat to exit Gaia Rock. Outside, you'll have a very brief talk with Susa, and then you need to go back to Izumo.
When you get back, the music has changed and everyone's lively, dancing around and all. You'll probably want to use the Inn, and possibly the Sanctum as well if you haven't already revived characters yourself. You may also want to improve or sell some of your equipment at the shop, so do that too if necessary. Once you're done, go back up to Lady Uzume's house (the big one at the top-left of the village).
After the discussion about Psynergy (it's very similar to the one you had with Maha in Garoh) you'll regain control of Felix. Before you leave, go up to Lady Uzume, and give her the Dancing Idol. She wanted to explore Gaia Rock on her own, and now she can, so she gives you Coal, the Mars Djinni as thanks!
When you leave, Susa will follow, and tell you about the legend of the Cloud Brand. Now, go back to where the Reveal symbol is (top right of the village). Complete it by moving the pillar one to the right, then use Reveal to make a ladder appear in the middle, which you can climb down.
Use Frost on the puddle, then use Sand to get on the other side of the Pound pillars (if you use Pound, you can't continue). Freeze this puddle, then use Sand to get back to the other side, climb the ladder, and jump across to the door.
In the next room, go right and down the ladder. Pound the pillar, then use Sand to get on the other side of the rocks. Go up to the pillar behind the rocks and Move it right one space. Go back to the entrance of this room and take the left path. Use the pillar to jump across, then go down the ladders.
Go up the right-hand ladder down here, and go all the way around, using the Pound pillars to get the Phantasmal Mail from the chest. Go back around, and now you can Pound one of the pillars to get past. Go up the left-most ladder and use Parch on the water. Go down the ladder and through the new door. At the end of the tunnel, you can get the Ulysseys summon from the combo tablet! Ulysses requires 2 Mars and 2 Mercury Djinn to summon, and it has an added effect of sometimes immobilising your opponent for a turn! Use Retreat and exit back into Izumo. Use whatever you need to, then leave.
It's a good idea to go back to Yallam and forge your Rusty Mace. It turns into the Demon Mace, a good weapon for Sheba. You might also want to sell some of the junk you've accumulated, although you can do this again soon enough.
Back on the world map, head to the beach to the west of Alhafra. Get off on the side closest to Alhafra and follow the path along to re-enter the familiar town.