dont do anything outside of your school. just do what your TEACHER teaches. I was doing other problems outside of the classroom and this made me get like 65% for my first 3U test. But when I studied what my teacher taught me, i aced the half yearlies.
Also MX1 shouldnt be that much harder than 2u course. I start "studying" for maths probably 3 days before the exam. I think that is good enough for you to ace the test. Also, dont just concentrate on 1 topic, you need to grasp the basics for all the topics first because stuff like half yearlies usually have 3+ topics tested.