You just have to prepare yourself and train properly for III DS. Gaining levels in III DS is easy and fast anyway, so it's no big deal. For the Sealed Cave I trained to level 8 in the left tower of the castle nearby and then the cave was a breeze. But I heard of a guy who grinded his Freelancers to job lv 99 and beat the sealed cave easily at lv 4. Proves how important job levels are.

The only time I really had to grind a lot was for teh Final Dungeon, but I wanted to be prepared, so I trained from lv 45 to 60. But in the end it turned out that lv 55 would have been enough, and the last dungeons were too easy for my taste. My own fault, though.