Although you have not provided the needed excerpt, here are key quotes from Book 12 that show Scylla could not be defeated.
“For Scylla is not mortal; moreover she is savage, extreme, rude, cruel, and invincible. There is no help for it; your best chance will be to get by her as fast as you ever can,” This kind was spoken by the Goddess Circe.
“Scylla pounced sown suddenly upon us and snatched up my six best men.” This shows that even his best men couldn’t have the chance to fend off the sudden Scylla that devoured them.
