Ext 1 Math
It can seem really hard at first but if dedicate some time for study and get help from teachers, friends, tutor etc, it get a lot easier. Don't think about dropped after the first month or so, or your first 3U exam.
Adv Eng
Basically just a deeper understanding of texts. Practice your essay structures and word-choice, learn to love Shakespeare, learn to talk about the area of study in an essay that's based on an area of study (e.g. If the essay is based on Journeys, make sure you actually talk about Journeys and solely the text itself, only use the text as supporting references.)
Physics
My worst subject in prelim. In my opinion, very hard and a big jump from year 10 science. Switch on from day 1 and make sure u perfect all your physics terminology and principles and get help when you don't understand something. Don't fall behind.
Chem
Sorry no personal experience, but I hear a lot of complaints about the level of difficulty so probably same advice as physics
Legal
No personal experience again, heard it's a lot of history within it.
SOR 1
Just content heavy, not "hard" to understand. Definitely my most relaxed subject but don't disregard it. It can be a fail-save in HSC if you don't perform well in your other subjects, since SOR is so much easier (IMO)