AND THE CONTRADICTOR OF THE MOTH AWARD GOES TO...... HARDCOREPOORE, for this stunning entry!
Its not a contradiction.
He's saying that Duriel is harder than Mephisto, Baal and Andariel, but not as hard as Diablo or Hephasto the Armorer.
Its like saying that grade 5 maths is harder than grade 1 maths, but both of them are easy.
The word 'hard' or 'easy' by themselves mean absolutely nothing. Only when compared to something else do they mean anything, because its just talking relative. Its a trick that advertisements use all the time. "We have the best product for cleaning clothes!" - best compared to what? Softdrink? Well duh.
Yeah, he's the only one that really lives up to his name. Anguish? Eh. Hatred? Eh. Terror? Kind of. The rumbling was definitely cool, and he is kind of scary running around like that. Destruction? Eh, we are more destructive than him by that stage lol.