Basically, the preliminaries of general relativity consist of teaching you how to differentiate things. Presumably if you've sat an A Level you can differentiate a scalar function of one variable, i.e. f(x). From there you need to differentiate functions of several variables f(x,y,z), then vector functions f(x), then you need to be able to differentiate matrices, and then tensors (which are like matrices, but even more so) and you want to differentiate them with respect to whatever the hell you like. Then throw into the mix the fact that space is curved, so the way you measure distances changes as you move around in space (how do you measure distance on a sphere? on a donut? on a coffee cup?) and then consider the fact that the way you measure distances changes over time, and in relation to the mass present in the space, which affects how the mass moves around, which then has another knock-on effect to the way that the space curves, ad infinitum.
As I said, I don't think you can learn it in a summer.