In my two attempts the most difficult subject i encountered was Islamiat.Although i don't know the syllabus of Islamic history and culture, maybe you should look into that for MCQs which are by far the hardest MCQs of any CSS paper.

Personally i used Dr.Arif Saleem(or maybe it's naseem) book, it is not a great quality book but it does cover all the syllabus.Prepare very well for Islamiat tbh, the last two papers were very difficult atleast in my opinion