I've just finished the AS course so I can't talk about A2 but I'll try to be as helpful as possible.
The main part about the ISA was past papers. It's the same questions over and over again, just worded differently so just practice practice practice! A notebook wouldn't hurt but I wouldn't say its essential.
Hardest maths was standard deviation and it's not too bad at all, once you've got the hang of it. Typically just percentages, rates of reaction etc. Basic stuff.
Hmmmmm, not that I can think of. There wasn't anything that I though was very hard to grasp, just lots to remember! If you do have trouble understanding things though, please go over it with your teacher until you understand it well. AQA LOVE "apply your knowledge" questions so if you all you do is memorize the info you'll lose plenty of marks.
I would say there is more content in unit 2 but not much. More subjects, less info in each subject, and a lot of overlap too. Some of the unit one stuff is needed for unit 2 as well.
I don't think you need to do anything before staring the course, although I would recommend buying the AQA revision guide. Very condensed info and was so helpful when revising, after looking over notes. It has lots of practice questions too with examiners comments on many of them.
General tips. DO PAST PAPERS! Use keywords, you can get the right answer and not get marks. The mark schemes are VERY strict. Just revise plenty and ask your teacher if you're stuck and you'll be fine
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