Hi Singh Satyam. First of all, thanks for contacting Careers360 to answer your query. 

First up, we don't know as to what constitutes 25% syllabus. You could have covered any topic in each of the subjects. If you feel confident as to tackling any question in the topic you have studied so far, then I would advice you to go through the Advanced Past years question papers and see if you can answer the subjective questions. If you are unable to answer even half or them, then sorry friend, you will have to do an independent analysis of the topics.

Note that the syllabus for JEE Mains and Advanced are totally different from each other. The former can be aced easily if you have a proper working understanding of the NCERT/CBSE syllabus of Class 11 and 12. But for the latter, you need to have an indepth understanding of all the concepts behind every subtopic of every chapter in every subject.

And if you feel that studying 25% is enough, just think that a similar analogy is that you have eaten 25% of your lunch, for example. Will you be satisfied? No, right! Same us the case here also. Not even 100% coverage will guarantee 200 marks in Mains if you don't have any idea of tackling the problem. Worse for Advanced.

Buck up man, the nation is already ahead of you. All the best! 

When did you give your XII Boards? 

If you want to drop a year, then believe me you have ample time to clear both. 

All the best!