Hi there! Current Year 12 student, so I should be able to help you out.
Your results are already pretty good, I too had around the same grade profile as you at this time last year, and turned into eight grade 9s and 2 grade 8s, so you're on the right track for sure. I'm also sitting Mandarin GCSE this year (!) so, I understand your pain.
For your maths and sciences, practice papers and questions are the best way to go - I can recommend savemyexams.co.uk or Physics and Maths Tutor - you seem to have grasped the concepts, the things keeping you from higher grades is exam technique, using the right terminology etc. Keep checklists and make sure you understand all the concepts.
For English Literature - mind maps and practice essays, don't be afraid to use your teachers to mark additional essays for work - try search for exemplar essays online, pick up similar ideas which you can also reproduce in the exam. Original knowledge is key to getting a 9 in English Literature. Your exam board should also publish exemplar essays in their mark schemes, so take a look at them, and see what they're looking for. It really does help.
Now, Mandarin - ooh.. this is a hard one. Make sure you have your characters nailed down, I've taken a look at this year's advance info and it seems fine - just learn your writings. Having sat 3 other MFL GCSEs, just do that and you'll be fine.
If you need any more help, just reply