In the time gap between Main and Advanced you should brush up your concepts and you should solve the questions which are of higher level.

You can also solve JEE Advanced specific books like for physics you can go for Halliday, Resnick & Walker and General Physics by I.E. Irodov.

For mathematics you can go for SL Loney and Hall and Knight.

For Chemistry you can refer to Paula Bruice Yurkanis

In order to score good marks lot of practice is required. So, make sure you solve as many as MCQs for your practice and complete mock test series to analyse your preparation level.

All the best!!