It might be a little easier if you wrap this in a macro:
#define NULL_TO_NIL(obj) ({ __typeof__ (obj) __obj = (obj); __obj == [NSNull null] ? nil : obj; })
Then you can write things like
fight.winnerID = NULL_TO_NIL([dict objectForKey:@"winner"]);
Alternatively you can pre-process your dictionary and replace all NSNulls with nil before even trying to stuff it into your managed object.