User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-99.152.5.112-20130505055952/@comment-131.137.247.7-20130509181741

I always took it to mean that celestial bronze and imperial gold were super effective against them, but it isn't like normal steel or other weapons are just going to pass right through them. The monsters are flesh and bone just like the anything else so if you drop a big enough rock on them they're going to die just like anything else.