You were close yourself, you just need to get the declared method and invoke it on the instance of the field that is containted within your object instance, instead of the field, like below
// obtain an object instance
MyObject myObjectInstance = new MyObject();
// get the field definition
Field fieldDefinition = myObjectInstance.getClass().getDeclaredField("f");
// make it accessible
fieldDefinition.setAccessible(true);
// obtain the field value from the object instance
Object fieldValue = fieldDefinition.get(myObjectInstance);
// get declared method
Method myMethod =fieldValue.getClass().getDeclaredMethod("greatMethod", new Class[]{});
// invoke method on the instance of the field from yor object instance
myMethod.invoke(fieldValue);