Here are some suggestion I remember for playing it on the PS1 and PSP (I doubt that there have been any drastic changes in the iOS version):
If you get all your characters to have the Squire's JP Up and the Focus skill, there's a simple trick you can do:
Get into a battle with some group with a chocobo. Taking out other enemies EXCEPT for the Chocobo.
Keep attacking the Chocobo until it's low on health (it'll look like it's crouching or knelling) and then try to corner, surround it your characters, and wait for it to use it's Cure to heal itself. Then have one character attack the Chocobo with a weak attack (enough to get it to low health, but not enough to one-shot it when its low on HP). Have the other characters all use Focus and then wait. You can continue doing forever if you want to.
Alternatively, you can use Black Mage with the Toad spell and cast it on everyone, but make sure you first kill anything immune to Toad. Then turn on the AI and let it do all the work. You'll most likely be left with a nice handful of levels and JP when you're done with the battle.