How to get exp to up my characters without the exp event ? In story? Is there a lvl to get lot exp reward ??

I don't know which stage is best, but stage 1.3 in Part 3 gave my units about 35k exp.

Currently the single daily event of Bonus battle exp 1 gives the most amount of experience per energy without the exp event but those are only available once everyday. After that, it second best place to gain exp is the Bonus Battle super exp but those are only available for one day of the whole week. The third best place to get exp is Part 1 Book 5 Chapter 6 on Hard mode. There you can continue gaining exp every day as there are no restrictions.