Complex sentence: He did a re-write of the script which he described as being long, incredible and awful which didn't changed anything in the film and even though he did lot of re-writes he hated the script and the film so he didn't want to be credited.
Split into simpler sentences: He did a re-write of the script, which he described as being long, incredible and awful, that didn't changed anything in the film. Even though he completed many re-writes, he hated the script and the film, and he didn't want to be credited.