wait till you are level 30 + and then go to forelhost without followers
or alternative any large dungon / cave with alot of falmers
Toughest Enemy? Maybe a Legendary Dragon. They're tanky as hell and hit like a sack full of bricks.
Karstaag is IMO the toughest nasty unless His weakness to Fire is exploited, or a character has very high quality gear.
Forelhost is IMO the toughest Dungeon because the opponents usually are at a level to challenge Our character.