Gotta go to the first town, go to the town square and take the "Listen to town crier" then eavesdrop, someones gonna tell you that the goddess is invinvible unless you have a special artifact, if you have reasonable charisma, he gives u a gem. Go to witch's hut in the mid tier area, she'll trade you the artifact for 1 of few things, including said gem. You can now fight the goddess.