Is it better to study for each section exclusively such as studying just biology for a while then chem and so on? Or is it better to study bio one day then chem on day and so on, alternating through and mixing the material?

Thanks in advance

i heard mixing was better

Mixing worked better for me. The reason is that if you mix different topics, you're constantly reviewing every topic every day; whereas, if you focus on one topic for one week and on a different topic the next week, by the end of the second week you've forgotten most of the material you studied during the first week.
But you should try both and see what works for you best!