Complex sentence: Gersen hopes that Alusz Iphegenia will be able to guide him to Thamber, but she has no knowledge of astrogation, and her only help is to be able to complete a nursery rhyme that clues Gersen into Thamber's location.
Split into simpler sentences: Gersen hopes that Alusz Iphegenia will be able to guide him to Thamber. She has no knowledge of astrogation, but is able to complete a nursery rhyme that allows Gersen to deduce the planet's location.