Personally I don’t think it is possible to crack bitsat in 15 days if you are starting from zero.
So I will assume that you are clear with all the basics of most of the topics.
First of all, from those 15 days, I will remove the last two days for final revision and mock tests. Try to give atleast 4 mock tests in that period.
Now you have 13 days with you. Go through all the topics and make a list of your strong and weak topics. Now from your weak topics, try to find out which topics are easy and you can complete in minimum amount of time. Proceed in this order and try to complete as many weak chapters as you can. Don’t worry if you leave a few topics. Try to complete most of these topics in 10–11 days.