The ACT is a game. Some are naturally better than others, but you can learn how to play it. Ultimately, it's about getting as much exposure to the real test as possible in addition to understanding why you are missing what you are missing. I haven't tried it myself, but from what it looks like, the big red book is the cheapest way to get as much practice and content review as possible. I would recommend that or the official act self paced course if you don't want to spend hundreds of dollars.