First, I only did OAR, I'm considering a lot of specialties but none that require full ATSB, so I won't try at all to help you there, other people know better.
Second, I used the Peterson's practice test and the practice astb the recruiter provides (the same as the basic one you see a lot here.) Before all this I did a few hundred SAT math problems, which was totally not relevant to the types of problems but got me a lot sharper on the basics. I started to realize pretty late how unique and not that numerous the OAR math problem varieties are. So focus on OAR materials. The best math resource is this flash card set. No explanations, use Peterson to learn how to do problems, then use this to practice.
http://www.proprofs.com/flashcards/tableview.php?title=astb-math-knowledge
Like you said, random, that's how the mechanical section was. You could study 10 minutes or study 4 hours and I feel you'd get the same score on Mechanical. Focus on math and just peruse the mechanical basics. You can't learn all there is in Mechanical I think. However if you're taking the full battery then maybe you should as there is a lot of overlap between flight sections and the mechanical.
I also used a flashcard for each problem type I would encounter on math and looked through them regularly. If you look at a problem and can't solve it, it goes on a flashcard. I'd like to repeat once more, the test gets a lot harder than the practice problems... If you know the basic frame for a problem though you can handle the complications.