AS maths is fairly straightforward, A2 is more of a jump up in the core units, but you should be able to do it if you can get an A at GCSE. Whether you should depends on whether you actually like maths or not. I did maths although I am not a natural mathematician, so I found it a struggle to get motivated and for me it was a case of rote practice rather than true understanding in the A2 core units, applied units were fine for me, but many people found mechanics very hard. It's very much a personal thing, some people find A-level maths very hard, whereas people who are naturally very good at maths and enjoy it find it easy.