No problem. I can tell you what I think helped me, but I cannot tell you what mistakes I made on the test because I still don't know my scores. Gonna do my best based on my standing.
First things first. The ARCO book is for the most part, useless. Believe it or not, it doesn't tell you very much in the way of information. The most I got out of that book was the format of the test, a study test, and a practice test. To tell you the truth, I did horrible when I took that practice test. It is good to learn how the test works and that's honestly about it.
If you want good information, use the Marine Gouge from Columbia that you talked about. That's some good stuff. Now you can use my method as a guide if you want, but remember I still don't know my scores, so what I did might not have been enough.
I printed out the entire Navy Gouge: physics, flight concepts, weather, the whole shabang. I read it all! Even if I didn't remember it, because I know I didn't remember anything but the main concepts (which is all you need for the test) They don't go into as much detail on the actual ASTB as they might lead you to believe. I also used the boating website to learn the boating "right of way", parts of a boat, signs and warning signs, etc. I had a few seamanship questions, the one's I couldn't remember really had to do with the signs. There were also some easy questions, like what part is the bow?
I went through the Marine Gouge twice, the one with the F-18 on it and has all Mech Comprehension questions. That presents foundation concepts for the MCT portion. Also, you will want to use the ASTB Prep-Test. I believe it is also on the columbia site. If not, let me know and I'll see if I can post it here.
It has bunch of things about the Aviation/Nautical portion and it was good for me to learn from. I didn't do so hot when I took it, but I learned from it and corrected my mistakes afterwards.
The most helpful material in my opinion: ASTB Prep-Test, Marine Gouge (mechanical guide), and the columbia website in general.
Now, as I've said at least three times, I don't have my scores yet. However, I don't think it was as a beast of a test as everyone makes it out to be. In my opinion, the study material I used was by far harder than the actual test.
If I had to give advice to study certain topics for each section, it would be:
1) Math/Verbal - algebra, geometry, and reading comprehension.
2) Mechanical - basic physics = simple machines, basics of fluid, liquid, and gas physics, equilibrium, they seem to like pulley questions and pendeulum questions. The fluid physics is focused on flight itself. How a wing produces lift, etc.
3) Spatial Appreception - this is purely being able to visualize in 3D. If you can't do it naturally, you better practice, practice, practice. I am able to visualize very well and so I missed 1/35 of these questions when doing the practice test. I think this section is the easiest way to boost your pilot score. It's quick, it doesn't require analysis, and it makes the test a little less grouling. Just remember to use the coast-line as a reference point, that's my best suggestion.
4) Aviation and Nautical - study the know your boat website from columbia. Learn about major events in aviation and space history and learn some Marine Corps history, i.e. who is John Paul Jones, or who was the first man in space? Also learn, flight concepts, parts of an airplanne and parts of a boat. I had some questions about clouds and a few weather questions. Just know the types of clouds and what altitudes they form at. There was a question or two on engine performance. You can find that material in the Navy Gouge. I think the most important thing to know as far as flight goes here, is understanding Angle-of-Attack, and how adjusting the control surfaces effects what the airplane does. This is purely a knowledge test. You know it, or you don't.
5) Aviation Supplemental - combo of all other sections.
Just remember, focus on core concepts and chill out before the test. Don't work it up in your head. It's all about knowing considerably more information, concepts, and methods than they can possibly ask you. Studying for this test, is for the most part, time consuming, not difficult. I hope this helps (...and I hope I didn't get like a 3/3/3 or something because then that would make me look bad right about now. Haha.). Oh, and don't forget about the Sticky's in the ASTB forum!