Apart from going through the book one more time, you can try to do a lot of hands on.
Dump your favorite
IDE and get on with your favorite text editor (not a
Java editor) - gedit, vi, emacs, notepad etc. Try to write a program from the scratch - put it in package etc., try to compile it and run it.
That way, you'll understand which class requires which imports, which letter is uppercase, which method takes what kind of arguments etc. and most of all, it will raise your confidence as you'll understand that you no more needs to rely on auto-completion of an IDE. Of course, the last step will take a little time - depending on the amount of hands on you do. If you are reading any dumps or question-answers, stop doing that - or at least try to answer the questions before you go through answers.
If the question contains code snippet, write it on your own (don't copy-paste it), compile it and run it (if you have any doubts about the answer).
Once you are comfortable with compile time issues and package structures(i.e. you are comfortably scoring 80%+ in those area), then only consider about using your favorite IDE. IDEs are great for debugging
Also note that mock exams in Sierra-Bates are quite tough than real exams. So, if this is your first mock exam, then it is not abnormal to fail

You can also take a look at what areas you are weak in.
I hope this helps.
All the best.