Sorry to hear you got stuck! I was very careful to make the encounter feel challenging without having a risk of players getting soft locked there. Guess I wasn't careful enough!
What level are your characters? If you're strong enough to beat the Lamia, those magic orbs should be beatable.
Earlier in the area, there's some water that fully restores HP, so make sure to heal up for free. In battle, you can use the Enemy Scan ability under Info to figure out the elemental weakness of the orbs, and use the magician to hit the weakness for extra damage.
Some good accessories for protecting against magic damage: Emerald Gem, Aquamarine Gem, Spirit Charm. Hopefully you've found some of them on your journey.
Let me know if you're able to beat the encounter. I may need to make that section of the game more forgiving.