Retaking an A level at university

Basically, I've recently contacted the university I wanted to apply to in order to get onto their graduate veterinary medicine programme and have been told that to have any real chance of getting an offer, I need to retake my Chemistry A level. My question is, has anyone ever tried to retake an A level whilst at university? I'll be going into my third year of my current degree in September and I'd really like to get my Chemistry grade boosted up as quickly as possible so I can apply to study graduate vet med in the next couple of years. Also, is it possible to complete an A level course in just the one year? And if so, would it be worth applying for graduate entry next year if I were to state on the application that I'm in the process of retaking my Chemistry A level? Oh, and lastly is it at all possible to just retake certain modules of the course? Or would I have to take the entire a level course again in order to improve my grade?

Thanks,
Gemma