Ok, this is what I did and i got to level 25 on the second day.
Do all quests until level 5 or 6. Get your gear updated.
Keep on doing Vagabond mission quests until about level 9 or 10.
Then from there, just keep missioning.
This game has a good system going, but the only flaw i see in this game is that the only way to level fast is to do mission maps.
It's to the point where as quests and field monsters barely give any experience. So it gets incredibly repetitive.