Hi, I got into the Mac.Robertson Girls' High School. I may or may not be the right person to give advice, but here it goes:
Do practice papers. I suggest doing an entire set of papers at the very start, resembling the exam. (It will help you to know how much practice you need.) You can do tests every couple of days to practice. I would also recommend doing an entire set every week to measure progress. You can find many practice papers online. In fact, I found an entire Google Drive folder containing tests from Henderson, James An, and Spectrum Tuition on this website. I can give you the link if you want.
Focus on key areas, keeping in mind that the exam won't (hopefully, because the test is quiet hard) go beyond the Year 8 curriculum. For example, if you know that you are weak in geometry, you could do questions based on that. Doing practice papers will help you identify areas where you need to focus.
Build a strong vocabulary for the Reading, Writing, and Verbal reasoning sections. In Verbal reasoning, there were questions that asked you to make an opinion/statement/argument stronger or weaker by choosing the correct option.
Do questions regarding problem solving. If you know most of the basic Year 8 arithmetics, you should focus on problem-solving. Many questions may be easy to solve, but they might be phrased in a twisted way.
Never give up. It's not like extra studying is gonna kill you.
Hope you get in!
