Kill all enemies except for a weak unit (like a White Mage) and then drink Potions until the desired level is reached. Stock up on Potions and then enter a battle.

If a unit uses a Potion on itself it gains 10 experience points. Healing abilities can be used when characters already have full health. This proves to be a great way to level up. Enemies will never get a turn until you decide not to use Smile. Then, go back and forth over and over again with this, as each turn it will net them 10 experience points. When it comes to their turns in battle, use Smile on the other Moogle Juggler, giving it the next turn. Also, make sure they both know the move 'Smile' before going into battle with them. If you're interested in getting two high level Moogles early in the game (or any time), make two of them become Jugglers of equal levels. You'll gain a random amount each time, but the higher the level of the enemy, the more you should get. Let them gather some experience, then use your skill to steal it. As the enemy hits you, that enemy will gain experience. Stand by the enemy, letting him hit you, while the rest of your team passes their turns. Also, make sure they have the skill 'Steal Exp.' Thereafter, go into battle (preferably a clan battle) and leave one single enemy left on the battlefield.

Have whichever character you want to level-up change into the Thief class. Then, steal experience! Quick Thief Experience

Allow an enemy to engage you and attack you, and then have the other character cast Slow or (preferably) Stop on the character. Couple your Thief in battle with a character who knows Slow or Stop. Make the character you want to level up a Thief, and make sure he or she learns the skill Steel EXP as soon as you can. Here's a really easy way to get experience.

Final fantasy tactics advance