I did not have sufficient opportunity to complete the Walkthrough of the Lava Ruins in time for IGN's last upload so I will post it here and you will find it in the Bestiary and Caves Guide probably on Monday.
From the entrance, you must go north into a large rectangular chamber. There is an exit at the northern end that is blocked with rubble. In the southeast, there are steps leading downward to the east.
Note the white urn to the left of the rubble at the far north first. This is your first bomb. Hit it with any weapon or charge towards it with a Monster Ally to trigger a detonator. You will have a few seconds to move out of the way. When it explodes, the rubble will be gone.
Each time you re-enter the Ruins, you will be forced to clear rubble again. Like any Monster Generators, blockages will re-appear at each new visit.
A single Monster Generator in the northeast corner spawns three Killer Ants and one Ant.
If you continue north after the bomb explodes, you will find yourself in a smaller area containing a single Monster Generator that spawns two Ants and two Killer Ants. If you walk to the northwest corner, past the Monster Generator, you will see a stairway that ascends back towards the south. Take it to find yourself on a narrow corridor that leads south to a small chamber containing two Elefuns spawned by a single Monster Generator.
This is your first encounter with the Elefun, a Monster who can be used for Transport and/or to water your Crops.
In the southwest corner of this little area is a sliding door. Go west through the sliding door into a small peaceful area with two crates and a tillable field containing two plots and a water source. You can grow any Summer Crops in the Lava Ruins.
This area is bisected by a crack in the floor through which lava can be seen. On the other side of the crack, you will see two Treasure Chests, but you will not be able to reach them here.
Retrace your steps thorugh the chamber with the Elefuns and north along the narrow corridor, then down the stairway.
You will find yourself once again in the area that held 2 Killer Ants and 2 Ants initially. Now go through the open exit in the northern wall.
As soon as you go through the exit, you will find a crate on the right that often contains Curry Powder. A crack in the floor to the north must be circumvented. The only Monster Generator is in the east and spawns two Ghosts. Go as far west as possible, then north to another ascending staircase that leads south again.
At the top of these stairs, you will find a bomb. Detonate it, then go west through the gap to another peaceful chamber. Here you will find another single tillable field with two plots and a water source as well as two white urns against the northern wall. White urns, like crates, can be smashed for items.
Look for a sliding door in the southwest and go through it. You will find yourself in a long rectangular chamber containing two Elefuns spawned by a single Generator. In the southwest is a sliding door. Go west through it to find a small chamber with three Rocks that can be smashed for Ores and a bomb. Incidentally, the most plentiful ore in the Lava Ruins, apart from the usual Scrap Iron, is Copper.
Activate the bomb, then go through the gap created in the southern wall. You now will find yourself on the northern end of the area you explored earlier that was bisected by a crack in the floor. Find and open the two Treasure Chests, then retrace your steps, as this is nothing more than a dead end. (Cooking Oil often is found in one of these Chests.)
When you return down the stairs, you will be in the area that had two Ghosts spawned by a single Generator. There is a gap that acts as an exit in the northern wall. Behind the Generator in the southeast corner of this chamber is a bomb.
When detonated, it will blast a hole in the wall. Go east through the gap to a small dead end chamber containing two real Treasure Chests and a Monster Box that resembles a Treasure Chest.
Retrace your steps to the chamber that had the Ghosts in it and go through the gap in the northern wall. You will find yourself in a small chamber with a single Generator that spawns two Ants and a Ghost. The chamber contains three Rocks that can be smashed for Ores/Gems.
There are two sliding door exits, one in the northeast and one in the southwest. Go northeast first into a long narrow peaceful chamber with two tillable fields, each with two plots. There is a water source here and two white urns as well but it is a dead end so return to the previous chamber and go through the other sliding door in the southwest.
You will find yourself in a very small room crowded with four Ghosts waving scythes, spawned by a single Generator. There is a sliding door in the north with a bomb next to it. Detonating the bomb may clear the room of a couple of Ghosts.
Go through the sliding door in the northern wall to find the isolated stairway to the second floor.
Path to the Shortcut Stairway:
From the entrance, instead of going north to detonate the first bomb, descend the stairs in the east to a narrow L-shaped corridor guarded by two Ants spawned by a single Generator.
A sliding door in the south leads directly to the shortcut stairway but you will be obliged to unlock the door on the 2nd floor from the other side before it can be used.
From here, you can go north as well past the Monster Generator to another sliding door that leads to a chamber with two Ghosts spawned by a single Generator. A short stairway in the southwest leads upward to a small peaceful area with two tillable fields and a water source. Each field contains two plots. Another stairway in the southwest leads upward to the chamber with two Ants and two Killer Ants that you would reach if you detonated the first bomb and went north through the gap.
In other words, both paths ultimately lead to this destination. From here, you can go north to the isolated stairway to the 2nd Floor.