The rank is all that matters, the actual marks for SACs are determined by your school's performance in the exam. If you remain at/near the top of your english cohort, perform well on the exam, and also get satisfactory scores in your other subjects, you will end up with a great ATAR.
Edit: In terms of how you can improve and what you should work on, I'd suggest practicing as many English pieces as you have time and effort for. If your teacher is a hard marker that may end up being a good thing because they will be able to push you to write really well. With that said, I'd also suggest getting a second opinion on some of your work, possibly from another teacher or from the english boards here at ATARNotes. It helps to get another perspective, because you don't now who will be marking your exam come the end of the year.