There is a final training area available only once a player reaches ExA level 3. 
The second requirement is that you have all of the Hunter medals that can be 
located in the three-star boxes, one in each of the seven first servers. 
You require a level 3 skill to open them. Once you collect all seven, return 
to the Low A room and talk to the training instructor that you speak to every 
time to increase your Hunter rank. He will then say that you have the opportunity 
to try the Master Course. It is incredibly difficult and lengthy. 
This is far harder than anything you will encounter in the normal story mode.