Giants, mammoths, invisible mages, zombie mages, umm I seem to have problems with alot of things... My Breton all-rounder is capable of pwning weaker human enemies and animals with one well placed intercontinental ballistic arrow, but those giants and their mammoths are pure evil, mages that can zombie other mages when I can't see them but I can see the body sitting in at the end of a one way corridor is bullshit its not bullshit that kills me but it drags out the fight.
At lvl 38 with lvl 50+ Destruct, one-hand, restoration, archery, and sneak I win.
But my only defense against magic is my Breton resistance unless I toss up a ward.