On the west side of the second floor of the ancient city, you will find an area with six faces shooting flaming arrows to block your path. After clearing the area of baddies, stand to the left (or right side) of one of the faces. Equip a semi-powerful and fast sword that will kill the face in one shot (lethal rapier should work). The faces respawn infinitely, and at 50hp a shot, it doesn't take long.