Weeping angels ambush from behind, however if and this is a big if the angels are not immune to influence fhe silent could put in their minds [you are always being observed] but i think the quantum lock process would reset the influence
With no prep, the Angels should take this. The Silence operate as behind-the-scenes villains working through agents; their actual fights although flashy also suggest slow movement on their part. The Angels only need to get in once to defeat an individual Silent. They're considered psychopaths and have demonstrated sadisitic tendencies, so even if they do forget about the Silence the instant they stop observing them I'd say that counts as them being bloodlusted and going for an instant kill as soon as they do spot one. The Silence's attacks on the Doctor have been far more methodical; I anticipate a similar approach to any attack against the Angels. My only question is the numbers.