I created two 'static' groups. I had the top 3 characters, and the bottom 3 characters. Each set of 3 had a leader (close combat) who stole first, a healer (w/ranged weapon) and a ranged attacker(either bow or gun). I would party with each group until one group was one level above the other group, then switch groups out until that group was a level above the other group and so forth. I took full advantage of 'Guests' and leveled my groups as much as I could while the guest was present in the party. This was also the time when I went after 'Marks' and Espers. At first I teamed the up with other characters of similar level until they were all the same level, then chose my sets of 3. It gets easier as you get farther in the game and you get accessories that increase your exp or increase your points for your grid (or whatever you call it...). I used that to even out the exp as close as I could, giving it to the characters that were behind and not to the character that was ahead. Worked for me. EDIT: you added the levels... I'd put a 99 in the party as damage dealer, but not the leader and two 18's with him, and take down high powered creatures and see how the exp goes. If the 99 is hurting the rest, then put the 35 in the lead and work them up from there.

You can't stick with one group; keep rotating the members of your group. I also repeat areas many times--you will increase levels, collect more items, raise licince points, earn more gil--it takes a lot of patience, but it's worth it in the long run.