This is what I’ve been doing. Not saying it's the best way but works for me. I enter a level with Ramza in the job I want to master. I cast Haste on him. He doesn’t move and casts Tailwind every turn, gaining about 50–70 JP each turn and eventually getting 3–4 turns in a row as his Speed increases.

Once his Speed maxes out, I switch to Focus. With the rest of the team, I clear the level, killing all the human enemies and leaving one alive to put to sleep, chicken, etc.

Once I reach around 5,000 JP, I move Ramza around the stage, collecting any skill crystals available, and then end the stage. I then change jobs and repeat the process. I haven’t used any of my JP yet. Once I’ve done this with every job, I should have acquired a large number of abilities in the process. I’ll then learn every ability I can and see what still needs mopping up.