I just hit chapter 5; is there a good place to grind up some job points?
You need a Dark Knight with Black Bane (30% of HP to deal group dark damage) and the Vampire support ability Self-Restore (restore all HP after a battle). Equip your best weapon (preferably with Two-Handed) and max out your P. Atk with equipment. You may need a speed-boosting accessory to outspeed all enemies.
Start fight, brave 3 times and use Black Bane. If you're strong enough you will rip through enemies in one turn. Self-Restore will restore your HP so just set Auto and have fun. If you get 6x Rafflesia and a Growth Egg (500k pg accessory from the Trader that doubles JP) you can hit 999 JP (Rafflesia are also Dark-weak). If you don't have the pg, a Freelancer support ability will give you 1.2x the JP (for that character).
If you don't have the levels, Firada/ga spam works as well on most enemies (except against Rukh who are Wind-weak). You'll need to restore MP though.
The counters to this are first strike; equip the accessories that reduce the odds/increase your odds of first strike.