The difficulty when soloing depends on the class you're playing..
Though it is always hard in the beginning..
I'm soloing as a cavalier for the moment, and after level 18, he is unstoppable!
As for other NPC's, it depends on if you're playing for the experience or role-playing, because a character thet decides he want to solo the game, probarbly doesn't want other people in the party, even for a short while
A thief/fighter rocks in my opinion
