Good solo's:
Roleplay a evil assasin or better yet is a fallen paladin.
Hard solo's:
Can't think of anything that trumps the strait Wizard Slayer (even with TOB) or the beast master. It however can and has been done befor.

Any class can be soloed it just takes the right player to do it. The real challenge is to roleplay and not just power game when soloing. Want to make it even harder, try soloing taking the first role on everything: orriginal stats and hit points. Playing like this adds a whole new dimension to the game.