There is another system, who use your actions in battle to increase your stats. If you're hit, your def will increase, if you use magical attacks, your magic stat will increase, and so on. But, it isn't easy to balance things out.
In my game, there is a level system to gain stats, but the progression depends a lot on the character. At equal levels, some characters may be 10 times stronger. Also, while level increase the stats, it is mainly the story that will give power boosts; ancient spirits, fusions, seals removed, and so on... My level system is made to make things easier if the player takes time to explore a whole map (as the enemies are visible on the map and do not reappear after being killed)