Well in terms of exam techniques, I'm sure you've probably been told to try and search for the general gist of topics, or even isolated words here and there, which would hopefully help to make the question seem less intimidating. In terms of developing your overall listening skills, all you can do is consistent practice really; unfortunately there is no magic formula or shortcut around this. If you immerse yourself in a lot of extra curricular French texts over time you start to develop your skills. French News is on SBS daily if you are able to record that, websites like
Zut and
Linguascope (providing your school has an account or you want to make one yourself) have a number of listening activities you can do as well. And if you are near to the city I believe the Alliance Francaise runs a few workshops throughout the holidays, so you may want to go check them out.
I had a similar problem to you, my reading and writing skills were really strong but my listening skills decreased since we left all the basic Year 9 and 10 stuff behind us. And I absolutely
deplore unprepared speaking, not to mention the fact that the conversation exam is conveniently placed right in the middle of Trials. I didn't really have the time to commit as much as I should, so I dropped French last term. The thing is with any language subjects is that you are up against quite a few semi-native speakers to which the practical applications of language in listening and speaking come much easier to them, so if you want to succeed in HSC French, or indeed the language in general, you will have to make ample time to study in order to improve your marks.