Get a job to level 99 ASAP (on goblins near Altar Cave) then I think you need to get an item from legendary smith. After that check the mognet, if you did the other prerequisites, this will unlock the ??? cave. THAT cave is the best grinding spot there is in the game, since you can only encounter dragons who give tens of thousands of exp. However, dragons are really tough, and if you run too close to the pedestal in the top part of the cave, it will start a fight with the Iron Giant, who will gut you at this level.
Also, it's a good idea to switch you characters to Black Belt for the grind, so that they would get extra HP from leveling. Especially Luneth.