My feeling in relation to that one is that it is a golden opportunity on a platter, and so the correct response from the GM is your sword breaks!Yeah, I mean, it IS possible for DW GM to say "fictionally that isn't possible", the canonical example being a fighter trying to hack a virtually invulnerable dragon with his sword.
If that's too hard, then a soft move - you sword bounces harmlessly of its hide, and it turns to breath flame on you - what do you do?
I think that the unintimidatable duke is like an invisible foe in combat - it's at the borderline between fair and unfair in terms of the players understanding what they are up against and what actions are feasible/optimal.4e clearly doesn't allow for that in COMBAT, but it seems like it is at least possible in other situations, as the canonical example is the unintimidatable duke (though that example has long been panned).
I think each table has to develop its own sense of where that borderline sits. I found that, over time, I was more prepared to push limits as the players got a better sense of what their PCs could do under pressure.