I don't think I would measure death curses in shifts. To me, that's something highly significant, it's a plot device more than anything. So I would treat it as such. Killing a player character with a death curse is boring and I would probably just say that the dying character just can't do that. Well, unless it's a player sacrificing themselves to kill the big bad, but not the other way around. Instead, I would go for something more dramatic.
DIE ALONE!
Agreed, Haru:
Except the power you can throw into a death curse is measured in shifts.
It's far less interesting, to me, to throw a 30-shift attack (or whatever) at players than it is to work
with them to come up with what happens to them when they're slammed with a Death Curse. Plus you have to pause play to do the math for the Curse, and then the math to stop it...that's a lot of not playing.
The numbers are there to make some sense of the game, not define it...that's what the narrative is for. If you don't want to do things by the numbers...you don't have to.
I've had...I think 4 different PCs take a Death Curse at one time or another. Only one took a straight blast to the dome in terms of shifts, and it was by far the least interesting and least memorable of all of them. Losing an eye, having an arm burned to a cinder, being cursed to never find satisfaction in life, those are interesting and fun. Other than the one instance of "blasted in the face," I worked with my players to come up with something that made sense given the narrative and their opponent that they thought would be fun, and they took an Extreme Consequence (at least) that was the "effect" of the death curse. And those have resulted in some great character development and RP for it. In Harry's case with DIE ALONE, I wouldn't fill his Extreme Slot for that but I'd have him change an aspect to fit the curse.
Death Curses are a big deal and don't happen every time a practitioner is killed, remember...you have to have a second's time, know you're about to die, and Harry implies you've got to be trained to actually pull one off, as in Wizard-level training. Those enemies don't show up all the time to start with, and when they do, they're often the Megalomaniacal Bad Guy who doesn't believe they could ever lose to some
meddling kids, and that darn mutt! So when the time comes for a real Death Curtse, they
should be plot devices. Every time one goes off in the books, it's a rather serious event--and a plot device. Even Simon Petrovitch's Death Curse is one....the dead Red Court don't make a practical difference at the PC (Harry) level--but they play a part in spurring on a Compel to throw Harry into another crazy adventure. One of my favorite developments in the entire series was
Harry losing the use of his burned hand for a while and everything that went along with recovering from it, physically and mentally/spiritually.
So my opinion is forget X shifts and just come up with something reasonable that will drive more good story because of it.
Now, to answer the question of how to Block something like that? Well, in practical terms, I'm not sure you really can. A Death Curse isn't something you can do if you're Taken Out--you have to do it, effectively, as a Concession. I guess it's possible to set up a giant block ahead of time in anticipation of the Bad Guy shooting off a Death Curse, but that seems far-fetched and unlikely...if you could do that, why didn't you just shut down his power completely from the get-go and avoid the fight and possibility of a Death Curse?
So if you're the one trying to stop the Death Curse-as-Concession, you have no mechanical way to stop it, practically, in my eyes--but you can bargain with the GM, as always. And that's where, if keeping the guy alive would be a big deal to the story or the PCs, "Cool! Let's figure something out..." And then we come up with a way to allow you to stop or block the Death Curse by invoking your HC and taking a consequence or three in exchange.
Maybe you do a cold-blooded takeout like when
Harry shoots Corpsetaker in the head
and take a Moderate-Severe Mental [Am I A Murderer?] consequence, for example. For the necromantic "stop him from reaching Death" style prevention you specified, maybe your PC starts thinking he's above the Laws of Nature/Magic and starts skirting the edge of Lawbreaking more, and you take a Mental Consequence or change an aspect to represent that. (I don't know the PC well enough to make a specific call, but I think you get my point?)
Whatever exactly happens...it should (a) be a Big Deal when someone uses or tries to use a Death Curse and (b) create
some effect that drives/creates more interesting story.