This way isn;t actually faster, but it makes you feel as if you're accomplishing more:
If you have a Black Mage/Magus/Sage, then use Bio on everyone except for the one you want to master their job. If that's not the most effective, then just have your main attackers beat the crap out of each other, not harming your star pupil, of course.
Once everyone faints except your preferred character, just keep guarding for as long as you like (I do 8 for the Sage, since 5 and 6 don't work), since a turn will fly by in 6 seconds maximum.
However, don't stay in an area like the Ancient's Maze...just go back to the beginning of the game, next to Ur, and keep going against goblins. If you shift your character's position to the back, then the goblins will never hit you more than 0, so guard as long as you like. Remember: job points come with how many turns you take and how you take them, not the foe you kill.