You will now need to reach the rocky area where you found Gil. Then go to this area, passing through the ice cave and the temple. You will need to reach the pond with a suspended platform above and a waterfall on the left. Go under the waterfall and proceed left, using Gil's bombs to knock down the brick walls. When you come to a precipice, drop down.
You will land in a completely dark area. A message will inform you that the first thing you will need to do here is to find the switch to turn on the light. Proceeding to the left, you will pass over some bricks. Stand at the left end of this brick floor and switch to Gil. Use his bombs to progressively destroy the bricks and drop down. The switch is located in the lower left corner of this brick area. After you find it, jump on it to turn on the light. Then use Gil's bombs again to open a passage and reach the door on the right. Enter it and you will find yourself again at the beginning of this area.
Proceed to the left and after the bricks where you found the switch you will reach another area with a brick floor. Use Gil's bombs to progressively descend to the bottom. When you are done, in the lower left corner you will find another Life Container. Then proceed to the right.
You will come to a precipice with a lamp on the ceiling and some small bricks suspended in the void. Use these bricks to reach the other side (if you have not turned on the light, these bricks will not appear, preventing you from proceeding). Be careful not to fall, or you will have to start this area from the beginning. Once on the other side, you can reach the chest on the lower ledge to get a Medicine. Then go back up, using Shades and Bop-Louie's suction cup to grab the upper ledge and proceed to the right. You will come to a point where there is a narrow tunnel that rises upwards. Every few seconds, rocks will fall from this tunnel, which can injure you. You will then have to be quick to climb, taking advantage of the Bop-Louie suction cup, taking refuge in the recesses on the left as the rocks fall. This operation requires timing. Once at the top, proceed to the right to encounter a very strange boss, but not too difficult to defeat.
This boss is divided into two parts: a white face that will move around the perimeter of the room, counterclockwise and a cat that will come out at regular intervals from the jar on the platform in the center of the room. For this battle you will have to use Gil, because the lower half of the room is filled with water. Jump over the white face to make it spit a ball. The best time to do this is as it walks up the right side of the room. Pick it up, and throw it at the cat as it comes out of the jar. Then, repeat the process. Like some other bosses, as you hit it, it will get faster, but if you coordinate your movements well you shouldn't have any problems. Hit the boss 5 times to defeat it and you will be awarded the Green Key, the second one. Only one remains.
Go all the way back until you reach the area with the lamp. Drop down the precipice and enter the door to exit this area.