The best area to train

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Actually, I'd suggest grinding up to level 50, because if you're where I think you are in the story, having your last Job skill and your full Job artifact set will be quite helpful.

Anyway, as for ways to level...
3.) Levequests. Not as good as the first two for pure EXP gain, but can be easily solo'd if you can't find a FATE party and you're getting frustrated with dungeons.

uhm, its the fastest possible way ingame to earn exp, you make like 20k exp/ minute, until you run out of leves
just do that single escort leve at mor dhona over and over, the escort takes 20 sec + some npc blabla = 1 min per leve and a 20k exp reward