Summarize the following:
Just like the rest of the characters of the game, you are required to go through a qualification quest before you can choose one of the three advanced classes. In order for the quest to be available for you, you must use Aisha and must be level 15 and above; otherwise, the quest won't appear in your Quest Log. Do this quest first before anything else. Enter the Underground Waterway under Very Hard Mode to collect 10 Dead Tree Branches and 30 Runic Power Crystal from the Ghost Magician and White Ghost Magician. The task is fairly easy, especially if you're in a party for a quick raid. This can be bought from the Potion NPC in Elder Village. This area can be found in the Elder Village map.  The rest of the qualification quest can also be cleared in the very same dungeon. To complete these quests quickly, enter the dungeon under Very Hard Mode instead of clearing it at low-level difficulty. You must kill 14 Jangos and 14 Kid Phorus. You must clear the dungeon with a B rank. You must collect 3 quest items from Bomber Mongs and 1 Special Nut from William.

Summary:
Get qualified. Infiltrate the Underground Waterway. Gather 5 Medium MP Potions. Enter the Suburbs of Wally's Castle.