Naxxramaskid
Everybody Hates Naxxkid
Blizzard in their eternal wisdom added 2 level 110 elite mobs with 7.5 million health (near the waterfall past first boss) that aggro even through stealth. While I had no problem killing them on my rogue (took a decent chunk of damage though) I can imagine that casters will have serious problems with those 2 mobs.