I use the ゼッタイ合格！ mock test books. They aren't easy (they're more difficult than the quizzes in my prep books), but I've only taken the JLPT once, so I can't really compare them that well. You get three of them in one book though, and they're pretty cheap; between those and the official mock test, you should be set.
In my opinion, the main difficulty of the JLPT is the time limit on the first half; mock tests help by getting you used to the format and preparing you for what the test is like, the rest of it is just having the knowledge and reading speed required to pull out the right answers quickly.
In my opinion, the main difficulty of the JLPT is the time limit on the first half; mock tests help by getting you used to the format and preparing you for what the test is like, the rest of it is just having the knowledge and reading speed required to pull out the right answers quickly.