Book Reports / How to write 20-paged critical writing on Lord of The Flies? [2]

Hello all. I will apply to a master degree program this year so I have to submit a 10-20paged critical writing sample for admission; However, I am not sure how to write. I am not familiar with "critical writing" as I am an international student. In undergraduate school, we used to analyse books in terms of characters, settings, summary, themes, messages and evaluation etc but I think what is required here is not like that. What I understand from my readings of how to write a critical writing is that I should come up with a hypothesis about my subject and then prove it. Is it right? Should the hypothesis be unique, by the way? Or what we did at undergraduate school works for admission as well? I really want to write a good critical writing sample, however I cannot come up with an idea which I can discuss for 20-pages-long. Could you help me finding ideas and give me some clues about how to write a 10 or 20-paged sample?

(By the way I want to write about a topic of Lord of The Flies or a work by Virginia Woolf)