For AS Mathematics
C1 (the first unit/exam you will take), is at the same level as an A* at GCSE, just a little more extended.
Statistics is also the same as the GCSE Statistics course. Nothing conceptually difficult.
C2 is an extension of GCSE. It may not be difficult to understand, but you may need to practise a little more as it requires more attention than C1. However, if you are naturally good at maths, C2 should be a breeze.
All in all, I believe that AS Mathematics is one of the easiest subjects to get an A in, if you have a natural talent for Maths.
For A2 Mathematics
The gap between AS and A2 is larger than the gap between GCSE and AS. A2 Mathematics is a little more demanding but shouldn't be a problem if you practise past papers. C3 and C4 are about the same in difficulty (this however is debatable). I absolutely hated Mechanics, even though it isn't really difficult to learn.
Overall, A-level Maths isn't too hard to understand, In the exam, they use easy concepts and use it in questions which seem to be complex at first, but are do-able with practice.