I just had a game that was quite easy, quite smooth, no real hiccups. And then the boss on level 6 sent an assassin to town. it was this skeleton warrior template - equal level to me, that did major damage just by being attacked - thorns and other miscellaneous damage in the high stakes, and it was very hard to hit and very hard to hurt. My magic did nothing to it, trying to take it head on proved foolish, even when the whole town attacked it - like eight or nine people, they had very little affect. And every one of them died.
That was a nasty combination of effects XP I think the only way it could have been nastier would have been major stone skin, or some kind of high level life leech.
So tell me of the nastiest town attacks you have had. Mass raids, or single entities, I don't mind which.