True, it would have been nice if FSM came with battlebacks.
I plan to solve this problem by supplying players with Grade Stamps. The exp curve scales quite nicely with the monsters too. There's a significant change in the required exp from level to level. There will also be quests where you go with specific characters only so that will hopefully give them added exp.
Lastly, leveling up doesn't really give that much stats. Most of the stats comes from equipment. Hopefully that solves it. ^-^