I think it's completely possible. My friend got an E in maths in his first year, retook all his as units alongside A2 and came out with a B. Completely possible, just need to work very hard. I'd suggest it's possible, it doesn't even matter what grade you got at GCSE. Work hard, set yourself a target everyday and do all past papers available. What I did with a level maths is that I did 2 hours a day and then went through Solomon papers first, especially with C3 and C4. Although C4 was hard, I found out when I did the proper past papers 2 months before i sat my exam; I was able to do it relatively alright. Go on exam solutions or khan academy, they're very good for maths. Exam solutions is tailored for a level maths tho, if you want to ensure the highest grade; do wider reading. What I mean by this, is look at different exam boards and do their questions. The more practice at maths with new and same questions, the easier the exam will be. But if you want to the whole a level in a year, ideally you would want to finish the AS content by November and A2 by April to give you time for past papers. You see people getting A* in maths, no it's not because of natural ability; it's because they revise effectively. To master Maths for a level, do every possible question which could come up in your exam from all exam boards. Good luck with your studies!