the best jobs kinda depend on what part of the game you are at. Some jobs like white mage are pretty much useful throughout the entire game, other jobs like Bard, are only really needed at one point in the game and are very niche the rest of the time.

Each boss has it's own trick on how to down it quickly, so even if you don't have jobs mastered, you'll still be ok as long as you figure out how to win. The game is pretty non-linear in that way as far as battle format goes.

Some basic gamebreaking tricks? Master Ninja and Hunter as soon as you get them on one person. Hunter learns an ability that lets you hit 4 times, and Ninja lets you use 2 weapons, so you can hit an enemy 8 times in one turn.

Another magical one, is mastering Red Mage (takes a long time) and any other Magic class, then using the Freelancer job. Red Mage's doublecast normally lets you use 2 magic spells up to level 3 (I think), but if you are on freelancer and have Time Mage mastered, for instance, that means you can cast any level of Time Mage spells twice in succession.