GCSE is really no indication of A-Level difficulty. Most poeple can get an A at GCSE if they have a bit of nouse but at A Level it's harder, a lot harder - Chem is one of the hardest A Levels about.
However, I really like it - as you can see, it's quite different to my other A Levels but it's a great subject, especially if you have good teachers.
I wouldn't recommend doing Maths, Physics, Chemistry and Biology A Level - it's far too narrow-minded, perhaps throw in an arts subject such as History, a language or English to give yourself a broader spectrum.