After you reach the end of the World of Evil painting, you must defeat the Metal Mantis to free the Village Chief.
Strategy
If you get too close to the Metal Mantis, it will become completely metallic, preventing you from hurting it, and it will start hitting you with its arm. Don't go past the conveyor belts or you will not be able to damage the Metal Mantis.
There are three conveyor belts in the Metal Mantis room. Two of the conveyor belts move toward the monster, while one moves away.
First, while staying at the bottom of the screen, walk to the side of a slow conveyor belt to make Metal Mantis walk to the side. Don't go too far to the side or Metal Mantis will turn and shoot a diagonal fireball at you. As long as you aren't too far away, Metal Mantis will just walk to the side to try to face you.
When the Metal Mantis is off to the side, run up the slow conveyor belt and slash at the monster, or just point your sword at it by holding L or R to cause continuous stabbing damage.
After a moment, Metal Mantis will spit three fireballs at you. Learn the timing so that you can get some sword damage in before you have to run away from the fireballs.
After running away to avoid the fireballs, repeat the process, making Metal Mantis walk to the side before you run up the slow conveyor belt to attack it. Eventually, you will kill the monster and clear the lair. Step on it to free the village chief. Return to Grass Valley and talk to the village chief.
Get the Brown Stone
The Village Chief will give you the Brown Stone that Leo gave him shortly before he disappeared. Then the Master will tell you that the Brown Stone is a key to Deathtoll's realm.
Get More Items
Before you leave, check the southeast corner of the goat pen to find a Medical Herb.
Go to the southeast part of Grass Valley to find a sleeping tulip. Push it to the left to find a hidden Pass under it. You can also use the Dream Rod on the flower if you wish.
Find the boy standing in front of the cave west of Gaudi the architect. Equip the pass that you just found and show it to the boy. He'll let you into the cave. Be sure to walk out of the boy's way so he can open the cave. Follow the path outside, then walk to the right side of the ledge to find a Strange Bottle. If you equip this item, you won't lose any GEMs if you die.
Go to GreenWood
When you go to the shrine and walk to the middle, the Master will tell you that there are six magical stones in the world, and when they are brought together, they will unlock the World of Evil (which is Deathtoll's realm, not the painting.)
The next stop is GreenWood. Step on the orange tile in the shrine and you will have the option to Move. Use this and you can move from the Grass Valley shrine to the GreenWood shrine. From the GreenWood shrine, step on the blue tile to enter GreenWood.