I have been preparing for the SAT Subject Test - Math Level II for the past month. Barron's is my review book and it is easy to understand when I am reviewing topics I have learned in school; however, for the topics we haven't covered, it is extremely hard to study those topics from the Barron's review book, which as the name suggests, should be used for review I guess. So I am in desperate need of sources to study from and some helpful advice. I appreciate any help!
Khan Academy can be a great tool for topics that you do not know. While they do not have a specific SAT II section, you can search the topic that you are trying to learn and watch videos and solve practice problems.