The Lamakan Desert is a huge desert on the southern end of the continent of Angara. Recently, the activity of monsters has caused it to become evil somehow, which makes the heat here unbearable. With Reveal, you can find Oases hidden in the desert to cool off.
The heat is formidable, but with your new power the desert is passable. You'll notice a new bar on the side of your screen; this measures the heat level of your character. If it fills all the way, the whole party will take a decent amount of damage. To avoid this, use Reveal to find an Oasis nearby. A series of rocks in a circular pattern is usually a hint of an oasis. The heat level will only increase when walking on the lighter sand, not on the darker rocks. Use this to your advantage and don't walk on the sand any more than you have to. As your bar goes up, the other members of your party will occasionally complain about the heat as well.
Head north on the rock as far as possible, then walk left on the sand. Go south once you hit the wall and you'll see 6 rocks. Use Reveal and hop in the first oasis. Go left on the rock again and stay on it as long as possible, until you see another rock circle. This one contains a Psynergy stone that's hidden. Head north from here to find a second rock circle; this time it actually has an oasis in it. Once you've splashed around, go left and then up the rock path heading north. There will be a split path; down the north path is a rock circle with a hidden treasure chest containing the Vulcan Axe. The right path has another rock circle, but Reveal will show a monster, the Ant Lion, in the center. If you're standing too close, it'll suck you into a fight. Heading north beyond this circle will show yet another one, but this time it actually contains an oasis. Head north into the next map.
In this room, walk north on the rock as far as possible, then go on the sand and continue north. An oasis rock circle is here. Heading left finds another rock circle, this one with an Ant Lion. South of the Ant Lion is an optional treasure chest (it only contains a Potion) so you can skip it if you want. Keep going north until you see several rock circles: the one on the left contains an Ant Lion, and has another circle beside it. In the one beside it is a Jupiter Djinni, Smog, which can only be seen with Reveal. After getting the Djinn, go back to the right (by now the heat meter is probably quite high) and head north a short ways to another rock circle. In this circle is a Psynergy Stone, so if you got hurt by the heat (or the Ant Lion) just use Wish several times until everyone is healed and pick up the stone. Head left around a cliff and then north to finally find the oasis. Keep going left until you see two adjacent rock circles, the right of which contains an Ant Lion and the left, a treasure chest with Lucky Pepper. Move south, past one rock circle (another Ant Lion) and then to another one, this time with a treasure chest containing 777 coins. Quickly walk left onto the rock and follow it south. At the end of the rock is an oasis, if you need it. Otherwise, go left through the door.
Next is a room with almost entirely rock ground, and sand "waterfalls". The current of these will push you down as you walk across them, so try to enter the stream from the northernmost point and run across as quickly as possible to prevent being washed too far. Head left across two of these, then continue left until you run into the wall. Move south and follow the path up to some sand. Once on the sand, you'll see a sand "waterfall". Use Reveal on it to show a hidden doorway. Enter the cave and follow its path to an exit. On the other side, walk left, then when you see another sand "waterfall", go north, climb a ladder and keep on until you hit the north wall. Walk left across three sand rivers. If you didn't get washed down already, climb down the ladder on the other side. You should be standing at a sandy area, with a sand "waterfall" pouring over. This one is hiding a cave too. Save now, then use Reveal. The Manticore is blocking your path.
Boss Battle: Manticore
Since this is the desert, most of Garet's Psynergy techniques are fairly worthless against it; the Manticore is quite resistant to fire attacks. Also, it will get to attack more than once per turn, so be careful of that. Have him use his support Djinn, Forge and Corona, but don't waste your time summoning. As normal, use Zephyr on the first turn to get a speed boost over your opponent. Having Isaac use Granite might also be a good idea. Mia's Djinn come in the most useful here, so try to get all 4 Mercury Djinn on standby as fast as possible.
Manticore loves using attacks which inflict status effects, so be ready to heal Delusion, Curse, and Poison, should the need arise. By the 4th turn you should be ready to hit him with two powerful (level 4) summons, which should wipe him out entirely. After it is defeated, enter the cave behind it and just walk along it until you find the exit.
On the overworld, take the northern of the two bridges leading west onto a small island. Wander the forest here for a while until you are engaged by a Venus Djinni. Defeat it to obtain Vine. In combination with Zephyr, Vine can be a powerful tool. Go back to the cave's exit. Head west across two bridges on the overworld map and you'll be in the next town, Kalay.