Awesome job level up
Here's what to do. Get into a battle and have each of your characters block for at least 5 rounds(Or 10 rounds depending on your character level). Afterwards finish your enemies off and everyone in your party should at least have one job level up.

NOTE: this level up trick does not overlap. Meaning you cannot block, let's say, 100 rounds and expect to get over a one job level gain per character.

I did this trick and I'm at a level 99 dark knight. This trick is useful if you want to get those last final weapons from the wandering smith.