-When you are in a battle, kill all but 1 enemy. With the remaining one, make sure that it can't heal, then drain its' life. It should flee to a far corner and stay there. Then attack your own characters and heal them consecutavely and you will get lots of levels.
-Have a Mediator use threaten on an enemy until he turns into a chicken, then use Speed, Power, and Magic Break on it. Now you can do whatever to get jp and exp.
JP & Exp
-Give your characters Gained JP up, AND the secondary ability of a Squire. Leave one enemy alive,then keep using Accumulate. I'd recommend making someone a healer, or a wizard that can cast don't move/act.
-Get the power break ability, put it on along with gained JP up, and kill all the enemies in battle but one. Surround the enemy and keep using power break so he is so weak his attacks won't harm you very much.
-Have two mastered Knights use power break on each other, and have your hero use yell until their speed is at the desired amount. They will go many more times then regular people, and you'll get fast exp.
-Get a character who has the frog spell and turn everyone into frogs and then turn all of your own characters into frogs. The frogs should hit way low and you will get about 20 exp and 40 jp for each hit.
-In a battle kill all but one enemy (Chocobo's work the best.) Back it up into a corner and continue attacking it while it uses choco cure in about 30 minutes you should have enough Jp to master your class.
-Get a mastered Bard and put him on a class that is hard to get jp and have him sing the life song while the last enemy left (that should be in critical) is attacking him.
-One of the most effective ways to get Jp is to make a party of 2 beginning Dancers, 2 beginning Bards and have them both dance/sing so it affects speed. After a while they will be super fast and can move several times before the enemy thus giving them the opportunity to master their class in this fight!
-At the Back Gate of Lionel Castle, make all of your characters Archers with 2-handed bows and the Time Mage ability "Teleport". Teleport them to Gafgarion and kill him and immediately teleport ontop of the wall. Next kill summoners and then you are home free to take potshots at the enemy.