When to go mystery room?

[Valar Morghulis]

Hi, im still at castle 3 and i dont want to over level. But when should i attempt to go mystery room?

If i attempt this room before final boss or something, would it make too easy?

Any ideas how to easily level up my other low lvl character aside going this room?

[Ederwolf]

I'd leave it for postgame as it is harder than the rest of the game. Play the game normally (make sure you went into the room and lost the battle on purpose to have this npc join you, cause she doesn't until you attempt the challenge at least once) beat the final boss, load the game and play around with the room then.

You can just pick your low level characters maybe like 3 max hide them in the back row and go fight the strongest monsters you can currently story-wise, they will catch up in no time because the exp you get depends on the level difference between character and enemy level and then swap to the next batch.