How are Airbus pilots trained for using the mechanical backup control systems?
According to AIRBUS but more specifically Airlines, have training syllabus.
Airbus Company states, clearly how, and how often to train Mechanical Backup Flying.

Each airline, which operates airbus A/Cs has to adhere with these recommendations and/or dictations.

Several failures of systems, or combination of failures can lead to the mechanical backup "state".

Every six months airline pilots are going to the Simulator for checking and training (LPC/OPC/SIM REC).


  Of course there is training regarding mechanical backup.


How often? 

It depends each time on the scenario, the seasonal systems to be trained, the systems failures, and many other factors.

Mechanical backup is a possible scenario by the Airbus, and well defined (but as always not exhaustive) in books, how can be happened. For this reason IMO all airliners provide this training to their pilots, and of course all the time (every six months) there is the possibility to fly a scenario leading to mechanical backup. 


  Generally the aircraft can fly and land with mechanical backup as long
  as the situation permits, the failures, the pilot skills, and plenty
  of other factors.


How difficult is?

1) Pilot Skills

2) Environment (Weather)

3) e.t.c (The list is non exhaustive at all.)


  Aircraft manufacturers and airlines want to give proper and sufficient training to their pilots, and professional pilots want to
  receive proper and adequate training, and fly remote and difficult
  scenarios in the simulators, in order to be ready for "anything" that would be happened.
  So to summarise, and once again to answer your question, in general
  terms, YES airline pilots get training regularly, on the Mechanical Backup
  Scenario to FLY and to LAND the A/C  and the difficulty is
  something generic, for some people would be less difficult than some
  other.