While I didn’t complete any tuition or coaching in preparation for the exam - so I can’t help you in that area - my best advice would be consistent practice under the exam conditions to sharpen your skills in preparation to write fluently.
I completed 1 creative and 1 analytical prompt each week. The most important part of this process is to be actively correcting and setting goals to improve after each attempt. You can do this by consulting a family member, (English) teacher, tutor, or even ATARNotes for feedback.
In terms of resources, here are some prompts/ guides I used (THESE ARE NOT MINE):
https://docs.google.com/document/d/1Fly9pFfObQZJWD--ZLF_RchgQ6ZtL_bzUMJt2YWy2wo/edit
https://drive.google.com/file/d/1sDIco6GKBGfbDG3GTeSNWd9lWg_XtqMd/view?usp=drivesdk
Also might want to check out some of the AN articles/ threads around information and resources specific to the JMSS exam.
Good luck 😉