SPOILER !
Lv 18:
Throw a stone statue into the first crack in the ground. Burn the bridge behind the waterfall with fire. Take the power of fire and jump down on the button. Wait for the elevator to reach the very end. Jump down, burn the obstacles with fire (the stone statue is used to bounce off it and change direction), until you can take the air power while falling, use it quickly to land on the returning elevator. Jump left from the elevator, there is a hidden path.
Lv 19:
Get downstairs. Place the stone statue on the poisonous slime next to the power of the fire. Go back upstairs using the fire. Take the power of water, stand on the button, wait for the elevator to reach the top. As water, fall through the chain and take the power of the air, jump on the stone statue, jump on the elevator and from the elevator, jump onto the platform on the right. There you will fly up to the key with the power of air.
Lv 20:
Use air and stone to place the statue on the poison sludge on the left side of the board, next to the elevator. Take the power of the stone with you. Use air power to land on the statue and jump left. There you have to throw the statue into the hole to the right of the button. Return in the elevator. Take the power of water with you and go as far to the right as possible. There, as water, you will fall down and reach the key by jumping on the statue you placed earlier.
