You can find all the important topics for the IPCC May,2016 in the practice manual that have been supplied to you by the CA Institute when you register for the CA Intermediate | CA IPCC. All the important questions are given in those books.
Also you can purchase the reference books on any website which contain the important questions which can be helpful for the studies.
They can also be found in free download notes which are available on CAKART in free download sections.
From where ever you see the important questions, all will be same as the important is important at every place and you can easily go through these important questions. You should see these only in the Practice Manual as it is easy source of study.
You should instead try to study everything as no one knows what will be there in the exam. Sometimes not- important questions are only given in the exam and then we have trouble if we don't know them. So try to do the complete syllabus instead of just the important questions.