To be fair it was unknown how to kill an Angel by everyone. Even the Men of Letters couldn't kill an Angel, only expel them from their vessel, prevent em from entering areas and most recently null their powers. The only time I remember an Angel being punched by normal people was when the woman had brass knuckles with enochian symbols on it.
I guess at the moment our best bet is the four Archangels and Death. Pretty sure if the cold can kill a Prince it can kill a Knight
Edit: Or rather
Archangels, Death, Leviathans, Eve(if she's part of creation, don't think that's been addressed since they bloody killed her off so quick).