1. Study in relation to the syllabus (most important)
2. Know all of your texts back-to-front
3. Have at least 4 related texts that you know well
4. Study with people that inspire you, and that are better than you
5. DO PAST PAPERS.....A LOT
These are probably the most important things you will need to do to improve on English. This is a lot to do, if you currently do none of it, but thats up to you if you want to change this. If you dont, fine, thats your life, if you do, even better. Start now, literally.
Probably the most important thing you will need is motivation. This is why you need to study with people that are better than you, and/or that inspire and motivate you. If you need any more info from me, feel free to PM me.
Lastly, to answer you question about using complex language, yes and no.
Yes, the markers do recognise the language you use, and will mark accordingly if you are able to use it successfully.
No, you wont get better marks if you dont know when, and how to use it. You need to suit your language to the form of writing involved. You will score high marks if you follow the syllabus and are able to use related texts and link them.