The best challenge so far that I had was going through the entire game ONLY fighting the bosses. I got to Zeromous at level 32. He was impossible at that level.

I went out and killed some side quest bosses (summons, chest monsters, etc) to get my levels up.

Now my highest character is 56 and im in the lunar ruins and have the game saved right before the Brachiosaur (spelling). Very fun game!

Soloing with one character does sound hella fun though. I guess ill try it but like someone else said, it doesnt take much skill to solo in this game. I personally think that if you have a party thats 20-30 levels under what they need to be to do a specific job, and you still win, that takes skill.