This page lists the locations of metal monsters that you can't defeat until you get the Zantetsu Sword.
The World of Evil Painting
Return to the World of Evil painting in Grass Valley, which is the area that you started the game in. You can quickly get there by using the right-hand tile in the Grass Valley shrine. This warp tile will put you in the room just before the Metal Mantis room. There are three Metagrinners to the east, then four to the north. Get rid of all of them and clear the lair to release ivy.
Go south along the conveyor belt to go to the previous room. There are four Metagrinners near the entrance, then two that are going around the room. After you kill them, go to the northwest corner of the room to clear the lair and release ivy.
Go south to the previous room. There is just one Metagrinner here, to the left of the stairs. Kill it, then go east to clear the lair and reveal a treasure chest. It contains the Tornado magic spell. This spell creates a tornado that moves at random in the nearby area.
There are no more metal monsters, so go south until you have escaped from the painting. Go into the cave behind the water mill (if the boy is blocking the cave, read the Grass Valley section of the walkthrough to find out how to get him to open the way) then go through the cave. Outside, go up the ivy and talk to the Blue Crystal Fairy to receive the Recover Sword. It requires you to be at level 22 or higher, so you probably can't use it yet. But when you reach that level, you can equip it in order to recover HP whenever you kill monsters.
Go to the little cabin south of Lisa's house, then go west and go up the ivy. Go to the left side of the ledge to find The Master's Emblem A.
Now go to the shrine and step on the top tile to move to Green Wood.
Go to the Fire Shrine. You can get there quickly if you step on the left tile in the Green Wood shrine, then go north to the Fire Shrine. In the first room, there are three Metal Scorpions. Two of them are going around the edges of the room, and one of them is going around a column at the top of the room. Their lair is on the west side of the room. Clear the lair to release a Blue Crystal Fairy. She will give you EXP.
Go down the stairs in the middle of the room. There are about six or seven Metal Scorpions going around the edge of the room. Kill them and clear their lair (in the northeast corner of the room) to reveal a treasure chest. Go around and open it to find a Strange Bottle.
Go down the stairs in the middle of this room. Go through this area until you find three hot paths going north. Go up the middle of these paths to find two Metal Scorpions going around the blocks in the center. Kill them, then go down the hot path. Go east and up the eastern hot path. There are three Metal Scorpions to the left. Kill them, then go north. There are two Metal Scorpions going around the loop of hot paths to the north. Kill them, then go west. There are three Metal Scorpions going around the blocks. These are the last ones. Kill them, then go north and continue following the path. The lair is north of the stairs. Open it to release a bird.
Those are all of the metal monsters in GreenWood, so go back to the shrine and move on to St. Elle's Seabed. (There are still monsters in the Light Shrine, but the Zantetsu Sword doesn't work on them.)
The only metal monsters in the St. Elle's Seabed area are on the volcanic island of Durean. (There were Metal Gorillas on Blester, but you could kill them earlier in the game using the Thunder Ring on the small pyramids there.) Equip your Bubble Armor and go underwater, then go southwest to the area with the glowing platforms. Follow the path to the northwest part of this area to reach the stairs leading to Durean.
In Durean, go right and go up the stairs, then go west. Use the Zantetsu Sword to kill the Metal Gorillas that come out of this lair. Clear the lair to release a mermaid. That was the only metal monster lair, so go back underwater and talk to the Blue Crystal Fairy to return to the Seabed Sanctuary. The mermaid that you released is in the palace. She will give you the Red-Hot Stick.
Continue the Game
Those were all of the metal monsters that you couldn't defeat earlier, so you can now continue the game in the laboratory or wherever you left off.