sir how should Ipreparefor jee advanced in last 20 days.

You must have given jee mains exam from there first analyse your weak part and mistakes u have done in the paper while attempting the paper such as -
1.wasted too much time on a particular question
try to avoid this type of miskates in jee advance
For better preparation revise formulaes in which u have problem and dont forget to revise inorganic and organic chemistry concepts...try to solve 2-3 mock test before the jee exam day