The math portion of the ACT test consists of problems that are very similar to the homework that your student has encountered in his/her math classes. The problems cover topics from Pre-Algebra, Algebra I, Algebra II, Coordinate Geometry, Plane Geometry, and Trigonometry. If your student is very accurate in their math work and has good recall of the topics they have learned over the last few years in math classes they should do well on the ACT exam. If they need some review over the material on the test, it is a fairly straightforward process to figure out where the gaps are and what they need to relearn/review. High School Math Live is certainly able to help your student to prepare for the ACT exam.
The SAT, which is generally preferred by universities in my part of the country, is a very different kind of test. The math portion of the SAT test is more of a thinking skills test. For the most part, I can work a section of the test in about half of the time that is given for the test without having to do any written computation on almost any problem. That said, I do NOT recommend that method of working for the testers - showing work absolutely is the best way to check accuracy! The problems have MUCH more to do with UNDERSTANDING and APPLYING math than with computing math. Students who run out of time on the test are, generally speaking, trying to do the problems in the wrong way. They are demonstrating a lack of understanding of the skills being tested.
By working through published tests with students, I am able to "connect the dots" for the students using the knowledge they have and connecting it with an understanding of how that knowledge applies to the problem at hand. I often hear this comment: "The test is so easy when you show me how to do it!" The goal that I strive for is to make it easy even when I am not explaining the problem to them! The biggest part of that is to help the student recognize how to START the problem. To do that, I show them how to recognize clues from the problem, to see the interconnectedness of their math knowledge, and to draw upon past problems that we have done together that make each new problem a little bit more clear.
Whichever test your student signs up for, certainly the slow and steady pace is the right way to go. Attending a seminar every week - or even every other week - would be an excellent pace.
Start early. Practice often. Let us help you to improve those entrance exam scores!