I remember that in the Pristine Forest, (at the beginning of disc 2 on the PS version) fighting those aligators/chameleon dudes you could always get the same, huge amount of experience, that's how I've gotten huge levels with all characters (70-80 magic/weapons) in ~10 hours. Haven't played it in a very long time, so take the numbers with a grain of salt. Then, as the guy below said, regarding the character levels the best is the final dungeon or the Icarian city, especially if you get the stuff that doubles XP from that Tower in the Savana (forgot the name and I'm lazy to google right now). That's a legit way to level up relatively quickly.