For people in the same situation as I was 2 months before, I want to show how I prepared:
I read the book Pro JPA 2.0. It's pretty good but not complete for the exam. I practised the most examples in the book.
Afterwards I bought Enthuwares question database for the certification. The questions are really close the real ones in the exam. If you pass an exam there, you will pass the real exam (if you haven't practised the questions before of course). What you should definitely do, is to go through all questions and read the explanations. I learned the explanation theory by heart, which is neccessary for the exam (for example, you need to know which exceptions will cause a tx rollback).
Afterwards I was able to pass the 2nd exam but it was close. I went through all the questions again. Now you should already be prepared to pass the examn, I would say.
I passed the remaining to with like 75%. I'm sure you will score higher in the real exam. There were only a few errors in the enthuware question database, which were fixed after I reported them.
The real exam also contains errors, so it's quite realistic :-)