When you first enter Lamarkan Desert you'll notice something different. There is a heat bar on the left, and if it reaches the top you will lose about 1/4 of your health on all the characters. To make the heat bar go down, you must find Oasases which are pulls of water surrounded by a circle of rocks. In order to find these pulls of water you'll need to use reveal. However, some of the rocks are traps, so use reveal when you're well away from them or you risk getting sucked into one and fighting a difficult monster.
Head along the path for a while until you reach the first circle of rocks. Use reveal on this to reveal a pool of water that you should jump into. Continue along the path and at the first branch off go up it to find another rock circle that has water in it. Head northwest up the narrow path, and take the eastern path over the western one. Don't go in the rock circle at the beginning of this path because it contains a crab ready to attack you. Instead, continue along in that direction to find a rock circle after the path starts heading north that contains much needed water. After cooling down, continue up to reach the next screen. This screen is much more open than the other screen, so you have to know which way to go. Stay south, finding in the first rock group a treasure box with a potion in it and in the next one a pool of water. After cooling down, head north a little and go around the cliff there. Shortly after going around that cliff you'll find another pool, this one containing a water hole. Just northwest of there is the entrance to the next zone. However, don't go there quite yet, as we have a Djinn to get! Head all of the way north and the start heading east. You're not going to be able to get enough water to sustain yourself, so be prepared to do some healing. Once you get to the eastern wall, head one group of stones south then start heading back West. You'll see a rock formation that has a full circle with a semi-circle on its side (it looks like this: c0) In the small semi-circle you'll find the Jupiter Djinn Smog, who you'll get without a battle. Now, start making your way back north, then keep heading west until you get to the wall where you'll want to head south. Once you can move left to the next screen do so, and you'll find yourself in an area with several sand waterfalls. Simply run across these sand waterfalls to the other side and they won't cause much trouble. Keep your bearings along that trail until you reach a "dead end" with a sand waterfall where you'll want to use Reveal to reveal a hidden door to a cave entrance. Head around this cave and come out the other side. Cross the sand waterfalls and get ready for a tough boss battle. Here you'll see another "dead end", but before using reveal save your game!
Now, use reveal on the sand waterfall to expose Manticore, who will scare you off. Approach him, press A and get ready for battle. As always use all of your summons against him right off the bat to inflict major damage. You have to watch out, though, as Manticore attacks twice every turn. So, you'll definitely want to just use Ply and Ply. Well with Mia each turn (if you have Wish or Wish Well you will want to use that instead of Ply and Ply Well). As always, use Ragnarok with Isaac, use regular attacks with Garret, and use Plasma and occasionally impact on Isaac with Ivan. Manticore has some pretty lethal attacks that pack a whollop even after the initial attack has been done. Watch out for his pesky poison and curse attacks that will take health away from you each time that character attacks. If a character is poisoned, have Mia use Cure Poison on him, and use a healing item or Isaac's heal on that character to keep him alive. Manticore has about 1500-1800 HP, so don't expect to beat him in the second round as you may done with previous bosses. As long as all of your party members stay alive you should be able to squeak past Manticore. When you beat him you'll get a load of coins, a Psy crystal and you'll be able to pass through the hidden passage that he was blocking thanks to Reveal.
Head through the tunnel going through the linear path. Head out of the door to get back on the world map. Head due north for a little, then cross a bridge to reach a small island. On this island is a new Djinn that you'll encounter in a random battle. Defeat him to get Vine, the Venus Djinn. Now, head south and cross the bridge going west. Continue going west until you reach the town of Kalay.