If you find it too easy, add Computers ( I think it doesn't matter what type they are, they're still counted as players ) and the monsters will have more HP ( and not sure - dmg ) .. I remember trying Solo 6 players with a Barb.. the Shadow Creature had 7k life instead of 1,5k O_O (it was before some weeks but I think it was something like that ) I killed it but the Skeleton King was the doom of me

Anyways, adding computers definitely helps to increase difficulty.