EDIT: Wrong thread looks like i got thread ninjaed. this was ment for the Ayaka question.
To reply to this: Go to your character abilities and look for the ability set option. there you can change sub job BUT NOT MAIN JOB.
Alternatively while preparing for a quest in the embark screen. Hold your characters portrait and will open a window where you can quickly change their sub jobs.
anoymous
Currently? Anything works.
For AI and Auto farming: Get green mage and learn Ruin but do NOT learn Baraero or Deprotect. This will make her AI only use Ruin to attack instead of useless buffs
otherwise: White mage = Spams shell and protect
Time mage = Spams Immobilize of haste if you have it.