In response to the common complaint of 'just take a sledge hammer to them', in 'Flesh and Stone', the Church soldiers were firing at them with automatic weapons at point blank range and didn't kill a single one of them.
That would indicate either Stormtrooper accuracy of the highest order, or the Angels are bulletproof.
As 10 said in
Blink:
Originally Posted by
10th Doctor
They are quantum locked. They don't exist when they're being observed. The moment they are seen by any other living creature they freeze into rock. No choice, it's a fact of their biology. In the sight of any living thing they literally turn to stone, and you cant kill a stone.[/
This would indicate they become a
fixed point in space, thus you can't kill them easily because they're both not there and their 'body' is a permanent fixture in space/time, which your mind interprets as a stone statue.