What grade in AS do you need inorder to be able to get a A grade in it at A2?

If you're talking about a subject which still follows the old modular approach, ie. where the AS is examined and forms part of the overall A-level grade, and assuming that the AS and A2 components are weighted evenly, then the overall grade will be more or less an average of the two. Meaning, if you want the average to be A, you need both halves to be A, or at most one of them to be high B, which combined with a high A might still give you overall A. If your AS grade is a low B, you would then need an A* in A2 for them to average out to A overall. Anything lower than B and you have virtually no chance of getting an A, without having to then retake some of the AS modules.

Generally you should aim for at least an A grade.