Complex sentence: Many critics believed it was a great advancement over Gounod's previous works, but one critic went so far as to state his doubt that Gounod composed it, which prompted Gounod to challenge the critic to a duel.
Split into simpler sentences: Many critics believed it was a great advancement over Gounod's prior works. One critic stated his doubt that Gounod composed it, which prompted Gounod to challenge the critic to a duel.