When you land on the Lofty Mountains, you find the entrance to the Palace of Darkness, where new Mana Power awaits.
In the first room of the Palace of Darkness, there is a Fierce Head. Defeat it and continue north. In the next room, defeat the Dark Knight. Go through the east door. There is a Dark Ninja in the path. Kill it, then go through the north exit.
In this area, there are some sludge-like enemies called Embermen. Use the Axe to cut the stalagmites to the right, then go south and use the southeast exit of the room. Go west and then use the north exit. Step on the switch in the cave room to create a whip post. Unfortunately your whip can't reach it from here, so use the south exit. Go east, then keep going south past the Dark Ninja to reach the next room. Go west and step on the switch. This opens the gate that you saw just north of the entrance to the Palace. If you wish, you can now get a Glove's Orb if you go back to the entrance and go north to a room with two Fierce Heads guarding the Gloves's Orb chest. If you want to do that now, just use the Escape Rope (which can be used unlimited times) and go north until you reach the room with the chest. When you're done, come back to where you stepped on the switch.
Now go east, down the stairs and through the door down here. Go north, fight the Dark Knight, then go down some stairs, then go west and then north. There should be a treasure chest in the north middle of this area that contains 1000 GP. If it's not there, try leaving this area and coming back.
From there, make your way westward and go through the southwest door. In the next area, go up the stairs and through the north door. Go past the Dark Ninja and through the next north door. This area has more Embermen. From here, you can use the whip post that you created when you stepped on the switch earlier. Use the whip to go to the right, then go north and step on the switch. This opens the cracked wall to the east. Use the whip to go to the left, then go south.
Get past the Dark Ninja and go south again. Go down the stairs and use the west exit. Make your way east and go through the south door. Go up the stairs and go through the north door. Get past the Dark Ninja and go through the north door. Use the axe to cut the stalactites to the west, then use the northwest door.
Now you are in a palace room with Embermen. Go north and then go through the north door. Step on the switch to reveal the outline of an invisible bridge to the west. Cross the bridge, then follow the path to a large area with Dark Knights. Go to the southeast corner of this area and step on the switch there. Cross the new bridge that you revealed to the east, then go north and step on the switch. Cross the new bridge to the west. If you're ready to fight the boss, go through the north door.
Lime Slime has 3470 HP. The Lime Slime is vulnerable to Salamando's magic. Primm can use Fire Bouquet, and Popoi can use Exploder. However, you should wait a moment after Primm and Popoi to cast the spells until you see how many hit points of damage you dealt. If you just hammer Lime Slime constantly, then the damage will max out at 999 hit points and you will be wasting MP. Keep an eye on everyone's HP and heal as necessary.
After the battle, you receive a Javelin's Orb. Go north to meet Shade. Shade gives his magic to Popoi, but Primm doesn't get any Shade spells. Go north and use the sword on the Mana Seed.
Be sure to go get the Glove's Orb if you didn't do that earlier. You can use the Escape Rope to return to the entrance, then go north from there until you reach the room with the chest. If there is a gate blocking your way, you need to open it by stepping on the switch just southeast of the large cave room that has a bunch of Embermen in it, as described earlier in the walkthrough.
Now you can go back outside and climb the Lofty Mountains.