With the compact, single boss layout of the dungeon, one single run isn't going to be enough to get Cain a level. Fortunately, he has Echoes available.
"I know how you two love a good competition, so I have brought you to another challenge. I want this dungeon cleared as many times a day as you can. Separately, of course. The exact details are up to you, and you are free to spice up the competition any way you like within those limits. Just give me daily updates on the score." Cain instructs them, and the Echoes give him the tentacle-wiggling Ancient version of a smile that keeps their teeth hidden.
That should keep a reasonable level of experience coming in. Not that Cain has a good reason to rush his leveling right now, but at level 290, he will be able to get the final skill in his Class Skill Tree.
[Ancient Horrors] Increase the quality of all Summons by 1 Rank and rename Basic Summoning Spells.
