Do all the hsc past papers; if you find any topic particularly challenging/difficult try to work more on those separately by doing textbook questions. Furthermore, the topics that tend to be considered hard in extension 1 can also found in extension 2 (eg. projectile motion, counting, circle geo, binomial theorem etc.). You can do these harder questions from extension 2 textbooks/papers to really prep yourself for anything in extension 1 from those topics.
Once, you're really confident, do the BOS trials and if you go really well in those then there's no way you're not gonna do well in HSC
Hope this helps
