(You require Snare and Ash, and must do this on the second visit to the forest) Go to the forest, as you enter go right to where the large boulder is blocking the doorway to a house, goto the nearby lake where the bomb plant is on a small area of land surounded by water.
Use snare to grab the bomb and then root him to the ground, throw the bomb onto the land opposite, once done use snare to grab the bomb again and carry it to the boulder blocking the doorway, drop it and use Ash to light the bomb.
Once it has blown the boulder away, collect the elemental fruit go into the house and open the cupboard to collect the skin.