No responses so far, so I'll chime in. I strongly caution you against preparing this way: cracking a case is a very repeatable process, so if you know how to crack a case today, we can pretty much guarantee you'll crack the one tomorrow and the day after.
If however you start learning to the test (prepare a specific case), you run the risks of (1) not getting the same case you prepared on, or worse, (2) getting a case that LOOKS like what you've prepared but actually different. And here, I don't think you will like the result.
In any case, the answer to a case is not necessarily as important as the way you got to it -> there is no shortcut.
My strong recommendation is for you to take on a variety of cases and learn how to crack them. That will serve you well, both in the actual BCG/McK interviews and in the (real) corporate world outside of consulting.
ex-BCG Dallas (and yes, from France)