Complex sentence: As a Watcher, Giles, extremely knowledgable about the creatures that Buffy must face is in possession of a large collection of books detailing the demon realms, upon which he relies as the main tool of research and problem-solving.
Split into simpler sentences: As a Watcher, Giles is extremely knowledgeable about the creatures that Buffy must face. He has a large collection of books detailing the demon realms, upon which he relies as the main tool of research and problem-solving.