I've been getting mostly A*'s and A's throughout year 12 and currently year 13, and i'd say that buying the stemup chemistry anki flashcards are extremely useful for getting the base knowledge as well as memorising answers specific to markschemes. This may sound repetitive but exam practice will genuinely be you best friend. Doing exam practice and doing the past exams really help. Make sure you can do the chemistry maths with good accuracy, as its a very big part of chemistry so if you've got that solidified than you should be okay. I'd defo recommend to use chemrevise for notes and chemsheets for maths topics eg. atomic substances as they've got whole booklets that explain the topics as well as LOADS of practice questions. I hope this helps, and i hope you do well!!