It's an interesting idea, but I don't like the idea of making accuracy irrelevant. That pretty much eliminates the usefulness of high rolls and multiple invocations.
That's why the beastie still gets a defense roll. That, imo, is the only reason you'd even bother rolling the attack at all. I mean, the thing you're attacking is literally bigger than a house, why would you roll to hit a house unless it was trying to get out of the way?
... Hm. Does anyone here know off-hand how much machine gun fire it takes to take down a house? Literally?