Written by [IGN: ngentot]
There is an alternative way to level up and get a lot of exp. Which is, to use the Akashic/rune stages.
This is great for 2 reasons:
a. You farm runes with tens of thousands of stamina anyways.
b. You can afk and it just maxes out your characters
Of course, it's not perfect, the slots are limited. You still need multiple core characters to beat the stage!
I do this for all characters, except for 1-2-3 star fodders (they are farmed in event 1-9 hard).
1. Look for 'non-essential' characters in your Akashic lineup. Usually these are tanks that need to just tank 1 hit and it's not that relevant whether they live or die.
2. Put in the devils you want to level up.
I attached an example of my exp farming lineup here - Akashic Black 10.
Both Brynhild and Baphomet are 5 star level 25 without runes. They are there to tank a hit or two and die.