I know I'll be torn to pieces, but Diablo 2.
Hell difficulty. Boss monster in Pandemonium. Probably it's Lord de Seis. What do we have here?
Enchanted Lightning. Enhanced Strength. Immunity to Magic. Immunity to Physical Damage. Enchanced Ice.
....Wait, what? WHAT? Not only the fucker deals tons of damage (Enchants), he can also be damaged by either ripping his immunities (I don't know if Physical can be reduced...), using Blessed Hammer (Paladin-only) or throwing an Iron Maiden and hoping he kills himself on minions. Which is very unlikely.
And what happens if this is your last boss? Sorceresses are screwed. So are Amazons, Druids, Assassins... And pretty much everything except for Necromancers and Paladins. I dunno if he'd also be immune to poison, but not everyone is carrying Poison Nova, the only spell with poison damage it's worth using.