I also did a critical essay this year. My advice would be to start off fairly broad, perhaps with an investigation of a genre, or literary paradigm, a particular author, or texts across time that you think share some interesting parallels.
However, after your initial 6-7 weeks of research, you should start to think about how you can focus down your work. A work that tries to cover 6 texts over 300 years, or a work that is too ambitious in its argumentation will unfortunately not work within the 5000 word limit that the Board has set. A lot of people feel disheartened at the realisation that there work cannot be as grandeur as they imagined it, but I think being concise and specific is important.
So; start broad, then try and specify to something smaller and nuanced within that. For example, if you started writing a critical essay about the similarities between three texts which span across two hundred years, you may focus this down to the re-appropriation of the anti-hero within literature and how they can be used to voice underlying societal issues (that's probably false, just an example).
I guess the question then becomes; how do you narrow down your argument?
1. WRITING- the act of writing is incredible important. Just try and write your thoughts, even if you haven't finished your research and even if you think what you are writing is rubbish. Through the process of writing, you will begin to realise what parts of your argument are the elucidating, and which aren't so successful.
2. Research- Research can help narrow your focus. In my own major work, research into critical theory allowed me to frame my argument through a lens of literary analysis, which inherently made my work more nuanced and focused.
3. One thing to keep in mind is the importance of STRUCTURE- a clear structure can help you focus your argument, and ensure that you don't waste time explaining tangential points about your texts that are not directly supporting your thesis.
Hope this helps