most the subject is easy (ie. the theory such as pseudocode, diagrammatic representation, social and ethical issues) , but there are very hard concepts you have to master such as search and sorts, I was averaging 80s in SDD in everything, and in my half yearly i got a nice surprise of 50% (well technically it was 70% because i couldnt do last section i came 30min late and missed out 20 out of 70 marks)
so when im studying for it, i make sure i put everything in depth, its difficult to sometimes connect the subject together, you can in some parts very easily
i recommend you do mind maps, dont forget SDD has a large syllabus t ostudy off