Mary's Seven Demons is a demon, so it cannot block while Three Nails is active. That being said, there still are many ways to knock out Three Nails.
Demons try to win via numbers/BTN, banding, conversion, discard, and removal. Angels are able to counter BTN with solid CBN, as well as BTN and numbers of their own. Angels can band like there's no tomorrow. Angels are very, very hard to convert since most specify 'human'. With Chamber of Angels and that silver recursion promo, discard isn't much of a problem. And removal can usually be interrupted when used by a demon. Plus, there is Victory and Might of Angels, as well as some immunity to demons. Thus, demons aren't particularly scary to angels, which is why most other enhancements focus on human evil characters.
