There's a few things you can do from now till the HSC to maximise your externals performance.
Make sure you have a thorough understanding of your text (themes, characters and their development, plot development, etc)
Do plenty of practice questions under strict timed conditions (normally, people either memorise an essay and adjust it to the question given, or they just memorise quotes and themes and then form an essay out of those; personally, I used to give myself 5 less minutes than the recommend (so for Paper B, I'd try writing each essay in 35 minutes to give myself 5 minutes before each essay to make a small plan and understand what the question is asking)
It may be quite difficult now, but try to get feedback on your essays (teachers, tutors, past students, students in your grade currently who are strong at English)
Don't be reluctant to ask for help and give help in return; HSC is about cohort performance, so everyone needs to back everyone else for the benefit of the whole grade