My friend, if you think ISCA a difficult subject, then you are suffering from one of the most popular myths available in CA student fraternity.
I would suggest you the thing which i used. Target for reading entire ISCA syllabus atleast 4 times and one last reading of entire book just before exam day.
You will be able to read entire book in just 6-7 hours in your last reading. If you do, i can assure you for exemption.
Regards