Diablo II with the newest update, the one with the "special" boss monsters that are invulnerbale to physical damage and all but one elemental magic? Normally, it takes a team to beat those guys (they have ALOT of HP (10-20K) also). - They ONLY appear in the Hell Mode. I assume you beat it in a;; 3 modes (Normal, Nightmare, and Hell) right? This was something new they added in V1,11 (maybe it was 1,12m I'm not sure) to get more people to play Multiplayer.
If you did, then my hat's off to you - that Hell mode in V1.11 is TOUGH!!!!