Is there a way to increase DC on slaying arrows?

[brian14]

Or on smiting, etc.?

[brian14]

Someone recommended cursespewing bow, as a general way to decrease target's saves. Probably good idea, but the question then is -- if I hit a mob with a slaying arrow from cursespewing bow, which effect comes first? If curse comes first, it will improve the chances of insta-kill. If it comes second, the DC on this arrow remains the same, but if it fails, the chances of insta-kill on next arrow are better.