Why are you storing a NSDictionary in Core Data? That is to put it simply, doing it wrong. If you want something like a dictionary just create a child object on the other end of a one to many relationship that has a name and a value.
If you design your model correctly then you will not have the problem you are running into now and you can use a predicate to filter and a sort descriptor in your NSFetchRequest directly.
Update
ohh thankYou ... the problem is the dictionaries i am getting from the web does not have a fixed Keys structure .. so I have to get the dictionary as it is as a transformable attribute
Still does not make any sense. The design I described is identical to having a dictionary inside a transformable object except that it is useful at the Core Data level. You do not need to have fixed keys in the design I described.
Rethink your design and this question becomes irrelevant.