-respondsToSelector: is a method on NSObject. Either assume that your id delegate is in fact an NSObject, and cast it:
[(NSObject*)self.delegate respondsToSelector:@selector(myClass:willDoSomething:)]
Or, better, make your delegate explicitly an NSObject:
@property (nonatomic, weak) NSObject<MyClassDelegate>* delegate;
Or make the protocol be a sub-protocol of NSObject:
@protocol MyClassDelegate <NSObject>