Complex sentence: His arch-rival is Lottie, who is not only a girl, but a girl who is more talented than he is and on rare occasions, he and the other boys hatched plans to get rid of her.
Split into simpler sentences: His arch-rival is Lottie, who is not only a girl, but a girl who is more talented than he is. On rare occasions, he and the other boys hatched plans to get rid of her.