1. Chemistry scales slightly higher, but not by that much
2. Both involve a fair amount of maths but the calculation questions aren't hard and are almost always free marks. For certain long-response questions in physics you need to be able to justify your answer with a relevant equation.
3. I...
Know what accuracy, reliability and validity mean in the context of secondary sources and also be prepared for questions that ask how you obtained your information. Apart from that be 100% confident with your content as they can still test the normal questions that come up in regular theory...