I wouldn't get too worried about it in Y10 (presuming all your exams are in Y11). 18/20 is still a very good (A*) controlled assessment mark.
I got an A* at GCSE (did it a year early in Y10). I did WJEC, so my course might be slightly different to yours. For listening comprehension, practice is important - past papers are one way of getting used to the spoken language. You could also find other resources online. In my experience, the speakers in listening exams speak quite slowly (at least compared to native speakers). If you practice listening to people speaking slightly faster than they do in the exam, past papers should (hopefully) become quite a bit easier. For reading (and this applies to listening too):
- Vocabulary is important - perhaps you could find a revision book containing the important vocabulary (I think CGP do good ones).
- Make sure you can recognise the endings for the different tenses (preterite, conditional, future, etc.)
- Do past papers (this might seem obvious) - Make a list of unfamiliar vocabulary you come across and learn it.
For oral and writing, all I can think to say is to prepare your controlled assessments thoroughly (in my school, at least, we write drafts and have them corrected by our teacher before learning them off by heart) and practice pronunciation - perhaps you could go over sounds/words that are difficult to pronounce with your teacher.
Hope that helps