Characters earn a bit of exp by completing dungeons. However, the best and fastest way to boost their levels is by farming expmons from Exp dungeons. These dungeons have no entry limit anyway so you can quickly max out several characters as needed. Feed elemental expmons to the characters sharing the same element as them. Rainbow expmons are universal so you can feed them to any character. The required EXP to reach the next level increases as the character’s level increases.
If you’re lucky enough, you may even trigger a Super Success. This will grant 150% more XP if you’re using a “material hero” of the same element or 200% if you’re using a “hero” of the same element. Each level up attempt costs a few Gold. It’s cheaper to do a batch upgrade compared to using a material hero in separate attempts. Make sure to mix and match various expmon rarities to avoid too much waste of surplus/exceeding exp.
All characters have level caps and you’ll be awarded with Crystals for reaching a character’s level cap for the first time for each tier, as follows:
4* - 6 Crystals
5* - 8 Crystals
6* - 10 Crystals
The game also tags a character you already maxed out before. Look for the ribbon icon on the top right of their portrait. To maximize a fodder hero’s potential, max him/her level out to get the Crystals as you evolve him/her on each tier until he/she reaches 6*. Once done, you can use that hero as a Limit Break material for another character.