For Zoology optional:

If you're clearing prelims this year-

-download PYQs (7 years at least), note down the frequency of occurrence of a particular topic

-mark the years on the syllabus itself

For example: Canal system (201x, 202x etc)

-you'll get an idea about which topics are "most probable" and which are relatively less probable.

Questions are asked in rotation. Spend a little time (5-6 hours, not in one go) cracking that pattern.