It is not wise to overlook the subject you are weak in. You will be able to qualify the exam in first attempt only if you have a hold on all the subjects. Hence, you must identify your weaker section and work on it.
Time Management
The main issue with most of the students is that they do not schedule their subjects in the beginning. Most of the students start preparing for final exams only after December which is wrong.
Formulae Chart
Prepare a small chart of all the important formulae and always keep it with you. Read them every once in a while during the day.
Practice
Get old exam papers, use special mock tests etc. to practice your calculation skills. Check for the long questions in the old papers and practice on them as they carry maximum marks.
However, the UP Board has removed Elementary Mathematics from HS syllabus.
While the change in the syllabus has been brought about as a result of the implementation of NCERT syllabus for the board students, the Elementary Mathematics was considered to be easier in comparison to Mathematics.