Just finished tutoring an Earth Science Regents student in June, he had failed 3 times before finding me. Finally, he made it. Below are just some tips I'd like to share.
1. Make sure you know how to use the reference tables very well. Most of the questions you need to refer to the tables. You can directly or indirectly find all the answers from the tables.
2. You can Google something called "100 ways to pass Earth Science Regents..." memorize as much as you can, they really work.
3. Also look for "Topic Specific Regent Exams". They are old Regent exams by topics, you can learn how many ways they can ask for the same type of questions. Just because you have no time to study all the topics, better be confident on some topics than none. So, pick the topics that you know from most to least.
Finally, good luck.