Unfortunately there is no topic that is not linked to other topics. Which means a particular Question can be a combination of any number of topics.
So no topic can be safely left.
At the max,
In Maths, topics like 3D, Permutation&Combination, Probability, Matrices & Determinants can have some individuality (But Questions from these topics will be based on other topics). So you cannot afford to leave out other topics.
In Physics, Modern Physics, Thermodynamics & Heat, Sound, Light and Waves and Oscillations can have individuality. But all of these topics are very important.
In Chemistry, everything is inter related.
So you have no choice of leaving out any topic.
So work a plan, and follow it.
We wish you all the best,
Ashwin (IIT Madras).