I think about 1/3 people get a band 6 for music 2, so it isn't too hard to get one
For performance: what you need to do is practice in front of people as much as possible! THis way you can get comfortable when performing infront of examiners and maybe even enjoy it. You also need to have pieces that you can technically execute that aren't too easy for you and that can show off your expression.
E.g. it would be hard to get good marks with a baroque piece purely because there was not much expression used in that period compared to say the romantic period.
You should also try to play off memory, as you can show more emotion and expresion towards the examiners as well as show them that you are able to play the music instead of just read the music (if you know what I mean) - if play well then there's no reason that you can't get 13+/15 for your performance
With composition - The school isn't allowed to mark your composition, but only the folio, so make sure you get that up to scratch for your assessment/trials!
FOr composition, you need to plan otherwise you wil have no direction and aim. After that you should write your composition, limit the australian compositional techniques for now, as you need your composition to make sense at first (can't do that with the shitty australian devices going on - of course if it has something to do with the structure or texture and such then use them as this is the basis of your comp). Once you have finished, add your techniques to make it more 'australian like'
I'm not too sure about the musicology elective, as no one at my school did it last year (or even did it in extension). However i'm sure that if the structure of your essay makes sense, and you do your research and know what you're talking about then you should be fine!
Sight singing - all you can do is practice! If you can't tell the difference between intervals yet then i suggest you get on to that asap.
What i did was I used to sing songs to help me remember (m2 = semitone (that's easy), M2 = diatonic scale, m3 = axel F/crazy frog, M3 = oh when the saids, P4 = national anthem, P5 = twinkle twinkle little star, m6 = one of my pieces that I used to play (can't remember the name), M6 = my bonny lies over the ocean, m7 = some weird interval that didn't fit anywhere, M7 = leading not, P8 = somewhere over the rainbow)
With the musicology exam, I don't know how to actually study for that! I didn't actually study and I usually did pretty well! Just know a bunch of terms you don't see too often (e.g. consord) - what I did throughout the year, is every time I saw a term i didn't know I made like a kind of dictionary.
Also know the concepts of music really well, however they've probably been drilled into you since like year 7 so you should be fine!
You need to have analysed at least 2 australian piece and 1 additional topic piece concept by concept
Good luck!
If you need any specific help, don't hesitate to ask