Accumulate experience points for the current level until the limit. Upon reaching 99.99% for level 50, talk to Otto Von Schroder. (http://aika-wiki.gpotato.com/index.php/Advanced Class)
Otto von Schroder: You're a gifted Aitan, you have yet to fail a mission. NakanoAzusa: Thank you. Otto von Schroder: You must've experienced a lot to be where you Otto von Schroder: are. You're only one step away from being a middle Otto von Schroder: rank Aitan. To change your rank, you must Otto von Schroder: accumulate an extreme amount of experience. NakanoAzusa: An "extreme" amount? Otto von Schroder: Yes, you will have to reach a point where you can't Otto von Schroder: level up. HiiragiKagami: How do you know when? Otto von Schroder: You will know. When you reach a point where you Otto von Schroder: can't accumulate more experience, come find Otto von Schroder: me. This is the last milestone for you! NakanoAzusa: Yes sir.
Reminder Dialog
Otto von Schroder: Now will be the most difficult time for you! Be patient!