I found an easy way that requires minimal effort. As soon as Robert joins the party, fast travel to the Old Monastery from the Map of Lemuria. Once you're there, fly up to the ceiling, exiting the building. Fly to the left and down the first cliff you see. You should see a gargoyle looking monster sitting on the edge of a building. Perform a surprise attack on him and you should enter battle with 3 weak monsters. Now just use Robert's Rain Arrows ability on the monsters. They should all die instantly.