Try replacing the bone walls with skull shields. Bone walls are relatively incompatible with forcing the opponent to spam many creatures (even if those creatures immediately die). I would also probably recommend swapping the bone yards out for more vultures, since a swarm of skeletons isn't going to do you all that much good as it stands, and you'll wind up with a full field very quickly (and therefore be unable to play any more sparks or vultures).
It's worth noting that you have to draw a spark for each nightmare, so you might want to drop the number of nightmares down a little bit to cut down on the amount of time you're sitting around with extra nightmares in hand and nothing to use them on.
If you want to go with something that is (to me) a lot more entertaining but probably even less effective, try to shoehorn some water in there, keep the bone yards and go for dry spell when your field is full and they have a good handful of skeletons. Then spend all the water quanta on ice bolts (if you don't immediately kill them with your gigantic vultures). Note this is probably going to work poorly in practice due to being such a complex combo, but it'll be neat.
If you want to go with something that is probably more effective, stick with the bone yards, drop the vultures and add SoP. This'll give you an actual way to do a really solid amount of damage really quickly and will also solve the running-out-of-sparks-to-nightmare problem.
