Jobbers solution is accurate but to simplify it:
You can do this easy as soon as you get to Gizamaluke's Grotto.
I was lvl 19 when I started. Go to the end of the grotto where the mogs are and climb a vine to the world map.
Simply use the battle assist
THIS DOES NOT VOID THE ACHIEVEMENT.
Start off farming in the wooded areas if the Grand Dragons are to tough. Bring a good supply of tents and phoenix downs just in case.
You will get 35308 xp if you have a party of 1 or 8802 xp and 2604 gil if you have a full party and want to evenly lvl up.