Granting that I haven't misread previous posts, there is a contrary perception that for for attacks dealing greater-than-normal killing damage, there is no indication beforehand that the enemy is set to strike. There is no preparatory animation placing the enemy in a ready stance. One moment the enemy is standing there with his hands in his pockets. The next moment you're watching as his warhammer smashes down on your head in slow motion, and you're dead. There is no set up. There is no invitation to think, "Hmm, he's set to swing his axe; I had better raise my shield." So, any investment you may have placed into blocking, or into moving lightly and quickly, goes to waste, because you cannot know it's time to try blocking and dodging.
That's because the killing animation wouldn't work if you had a chance to evade before it struck. It's a sequence that plays out in a particular way. When the animation kicks in, it's already a done deal. And it's already a done deal because the game already calculated you got hit. You don't see them about to swing because that's a part of the animation. The game calculates they swing and hit you, but you only see that swing as a part of that animation because that's what the animation is about.
In exactly the same way -
You click to make a swing
The game calculates that swing hits and is fatal
Killing animation triggers
You clicked just before you actually see the swing (you really see this with two-handed because it's a more drawn-out animation) - the time delay in clicking and seeing your warhammer actually swing and kill them is to allow the animation to play out - when an enemy does it to you, the game already decided they made a successful swing. In effect, you're already dead - but you are left with an illusion you could have dodged because your character remains frozen for a second for the huge swing to play out. but the fact the animation is running means you already failed to dodge in terms of what the game has calculated - the enemy, as it were, left clicked their mouse slightly before you see them hit you.
that's how I see it anyway. It's comparatively rare any enemy manages to pull this on me, even stronger ones that can and do kill me in the regular way with big warhammers. So the game is, to me, clearly satisfying a bunch of variables before it allows it to happen. And, like I said, if I do it to an enemy the actual blow falls after I clicked to make the blow (only noticeably with things like warhammers as swords are so fast) - so it seems logical the game calculates the enemy did the same.
Whatever the case, it happens so rarely (to me at least) I can't see it as any kind of major issue. I do it to enemies all the time - the fact they occasionally do it back to me is, as far as I'm concerned, simply giving me a taste of my own medicine once in a while.
The moral of the story is if you're fighting a strong enemy with a warhammer, don't give the game a chance to (metaphorically) register the enemy left clicked their mouse, register you're in the way of that blow, register it is fatal and trigger an animation that gives you the illusion you have plenty of time to dodge when as far as the game is concerned you're already dead. I can't stress that enough - when an animation triggers you are already dead in the eyes of the game. You're seeing something that as far as it's concerned already happened.
The issue is, like you say, you don't actually see the blow coming. The game simply calculates it happened. Them's the breaks. You don't see it coming when a dragon grabs you and does its killing move. You perform killing moves all the time and your enemy has no chance to avoid them. So it gets thrown back at you once in a while. So what? Learn to minimise the chances of it happening and accept what you dish out can be dished out back at you if the game calculates certain variables are in place.