To unlock this achievement you will have to do two things. First unlock all the beginner, intermediate, advanced and specialized jobs. There is a guide with each job and their requirements made by DarkClaw3050.

The second thing is the game counts each playable character unique class job as part of the achievement. So after you beat the game you continue with the DLC content. There are three separate DLC events, the first two are one battle each involving Urashima and his friends and then Aome and her friends. The third DLC unlocks Momotaro and his friends as playable characters and if you have unlocked all the classes listed in Darkclaw guide then you will receive the Job Guru Achievement.

here is the link to DarkClaw3050 guide to unlocking all the generic jobs: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2305248874