Über is originally a german word with multiple uses. Translated to english I would say above or over.
There might be more opinions on the exact meaning, when used in games. Especially on other languages. But I would say it's used to descripe something very superior.
He's a joke compared to Duriel or Diablo, always has been.
I don't agree at all. Actually all random bosses could be insane, depending on what effects they have got. Multiple shots and lightning enchanced is in my opinion the worst thing to encounter when playing HC.
With a HC character I would never try to kill Hephasto with these 2 effects and extra fast. Duriel and Diablo is even though they are very strong, no comptetition to that because they are predictable.
But the hardest ever, was in Diablo II classic. It's been a while since I have seen his name but I am pretty sure it is Hephasto. The one guarding hellforge. I don't remember if he still get random effects but once he did and no matter what he has definitely been nerfed a lot since the beginning. I will never forget that maniac, extra strong, extra fast, teleport hunting one through the entire river of flame, healing when teleporting.