Not being sure about advanced is perfectly fine. This is actually really good because now you can concentrate solely on BITSAT. I think you have got a good hold on the basics given your board percentage.
Now if you make use of these two months properly, you can easily crack BITSAT. I’d recommend using the big Arihant book as it has lots of questions on each topic and it also has past papers. The questions are not very tough to solve so you can solve all of them. See to it that you solve all. The size of the book can make you nervous but trust me cracking the questions will be really easy. Some questions may be too easy, but keep grinding, it will all pay off.
After that you should solve all the papers given in the book and also other BITSAT papers which ever you can find. The questions are good and they test you similar to the exam itself. Rather than trying advanced problems, pratice on improving your speed, accuracy and clear knowledge of the concept and its application.
Besides doing these, you can solve Mains questions also. They will be a bit more conceptual and will help in clearing petty doubts and concepts and eliminate the possibilities of silly mistakes.