The worst things I've thrown at my players have been a dragon built to swoop down grab the players fly off and drop them again. And a Drowned with either 5 or ten swordsage levels focused on setting sun throws and shadow Hand teleports as well as a concerted attack of Mind Flayers and thier pet Balhannoths. Oh and a Dal Quor Planar Sheperd and his family of a TOB More Dakka Archer, and a summoning sorcerer.