To do this you need to defeat 2 Mutant Hellions, they are Stheno and Euryale. After defeating both of them, visit Pendrago Church and speak with the priest (with “!” on his head). Check the bookshelves nearby to learn more about the Gorgon sisters. Keep reading the book until Sorey decide to visit Gorgon’s sisters home at Glaivend Basin.
Continue the game until you’re able to explore Glaivend Basin again, go to the northwest part of the Glaivend Basin. You will arrive at another abandoned village, you should find Stheno and Euryale there.
The battle should be very easy at this point, since it’s been a while after you fought them. However they can still petrify you. To kill them easily simply enter the battle with max BG on all characters, and then use your armatized mystic artes. Their HP are only about 40,000 each, which is low at this point of the game.