The Litwick Event requires you to have at least Surf. Once you have the HM, go back to Amethyst Cave and explore the cave, specifically the area of water near where you fought the Galvantula. Be prepared for tough battles, as one instance will require you to fight two battles in a row.
For the Dusk Stone, I believe you could get it in one of the green bushes near the entrance towards Goldenleaf Town.