A candidate can give JEE Advance if and only if he/she qualifies for JEE Mains. 

JEE Advance cannot be given if score in JEE Mains is very low.

So both of the exams are important. If a student is not able to clear JEE Advance then he/she can still go for engineering by score secured in JEE Mains.

I would suggest to focus on the syllabus, problem solving and remembering the formulas, knowing the concepts. 

If you still have any queries feel free to ask in the comment section down below.