simple.
very simple for spellcasters.
go just a little outside of Seydaneen (spelling?) and cast magic. it levels without actually having to come into contact with anything. just through use. focus on the desired spell. when your mana runs out. rest.
repeat.
sure you will witness a blight storm rather early on.... but you will pretty much be able to pwn any interruptions and etc after a short amount of mindless spell spam.
now. my answer to the topic n_n"
I was going to say Makai Kingdom (just started back new from today, actually I never got far in it... but now I am in Chapter 4 n_n! 100k+ and a level 20 something leader) but with a little luck (Charge Spear Reward at level 9! = +100 HL atk) and finding this... amazing map where all you had to do was walk into a castle. and reap the rewards at the end. I am pretty much set through it.
but... knowing the level cap. and knowing that this game will always challenge you no matter what (AND with improved AI and overall gameplay) you will always be challenged.