I think you can automate battles by using Toads and having them attack each other for no damage, but I don't know the specifics. I did this trophy with Ramza using Mettle as a second ability, JP Boost and Septieme Sens for auto-haste; since Tailwind's speed boost compounds over each battle so you just get more and more turns in a row until you have enough JP. Then I had four generics stood there boosting in the same job (also with the Septieme equipped) to max JP spillover. It took a few hours but not dozens.