Hi Satwik,
I think you should definitely start with Campbell. Make sure that you google the things which you find confusing rather than leaving them. Personally, google was of most help for me. Remember that reading Campbell completely is a huge task, so I suggest you start with the Genetics unit, then move on to Ethology(behavioral biology) and then read Cell Biology from it. In my opinion, if you have less time then reading anything else from it will not be that beneficial, rather you should solve lots of practice papers such as NSEB, INBO, Toronto Biology Olympiad, USABO, ABO, NZABO, etc., etc. These papers can be overwhelming at first but trust me, don't focus on the scores, rather focus on how much you can learn from analyzing the options of the questions....I will suggest you start from Toronto then move on to NSEB then on to USABO Opens, then USABO Semis, and finally INBO but it depends on you how you want to do it :) Google whatever you are doubtful about, teachers aren't of much help in olympiad preparation but trust me if you do all this, you will definitely reach at least the camp :) And for later on you can read Raven's, Sherwood, Lehninger's, Albert's, etc. but I think you should first read Campbell and solve all the practice papers mentioned above :) Hoping that you get the gold :)
Sincerely,
Ravi.