Okay, we're going to get the final Crystal (and the jobs it unlocks). A little earlier than was predicted, but no complaints here right?
Fly to the northeastern continent. Before, you couldn't explore because of the dreadful Guardian Statues. Now, with the Fangs, they'll let you by. At the statues, land and walk down the row of guardians. Nice to know that they won't shoot you down or anything else harmful.
Once they disappear, return to the Invincible and fly deeper into the mountains to the tower surrounded by a wall. Go inside to the Ancients' Maze . Go due north within to find the Crystal's Chamber. As you prepare to use it, you'll be jumped by a surprise boss fight!
Similar to the boss beforehand that you just fought, Titan doesn't necessarily have anything too exceptional up its sleeve. So just handle him how you handled the previous boss and you should be good. Since you had access to the Overworld earlier, if you aren't doing so good you won't have to trek through another whole dungeon again to have a rematch.
After the battle, you'll watch some more scenes and receive the light of the final Crystal, the Earth Crystal. We now have all of the jobs. Lets take a look!
Black Belts are like super-Monks. They have a Boost ability that can be used to double or triple their offensive power. They'll halve their own HP if it is used more than twice in a row, though.
The Magus can use all Black Magic. The Black Mage, ironically enough, could not.
Devouts can use all White Magic. Similarly, White Mages had restrictions. Wonder why they call them "Black" and "White" Mages if they can't use every single Black and White spell?
Summoners are able to use the super-special effects of the Summon Magic spells you've found recently. These effects are quite spectacular, to say the least...
Sages are able to use any spell you wish. They just won't get the super-special effects from Summon spells that Summoners will have. That is, after all, exclusive to Summoners.
Ninjas can use Dark Blades, just like Dark Knights. But what makes them special is that they can also Throw weaponry. Shurikens are such an example, having 200 attack when tossed. At just Job Level 1, a Shuriken can deal 6,000~9,999 damage! You can also throw other weaponry! They can deal a lot of damage with a high enough Job Level, making them worth investing in.
If you want a detailed writeup and analysis, plus recommendations, check out the Jobs section of this guide.