Operencia: The Stolen Sun

Towards the end of the World Tree area you will find a floor covered in hungry plants. First you need to go underneath them (there's a vine hidden behind some grass before reaching them that lets you climb down safely) and use the Magic Feather to lift the wooden logs underneath the mouths to close them. You can't reach one of the mouths so you have to progress first and come back. From the entrance, line the logs up horizontally in the very middle of the room and when you climb back up you can cross safely.

Later on in another cave you will find a wooden log you can pick up. Take this back and you can prop it under the mouth in the corner that you normally can't reach. Now use the Magic Feather again to carry a log under every single mouth at least 1 time. I waited a second under each mouth until it finished the closing animation just to be sure. Achievement will pop once you have closed every mouth at least once.

Here's what the room underneath looks like and the solution needed to cross for the first time.