I did a German qualification overseas, considerably easier than A-Level but harder than GCSE, equivalent to AS maybe and got 39/40.
The trick for me was to learn as many words as you possibly can, while at the same time mastering the grammar rules. This will give you a mechanical understanding of German, but not necessarily the kind of understanding you need to be able to converse in German. Once you have this kind of understanding, watch as many films as you can, talk to as many Germans as you can until you turn this mechanical understanding into a fluid understanding of the language.