Personally I think this can be very broad, maybe a little too broad evne for a major hermetic virtue. Although part of the RAW I'd have issues with it as a GM. I'd probably want to set some perameters.
However this is purely a game-feel and Personal taste arguement.
I agree that some parameters are necessary, but I don't think the line is hard to draw.
As a major focus, it's either Perdo spells of Momentary duration which cause immediate damage (Wound that Weeps, Obliteration of the Metallic Barrier), or Creo spells which create immediately damaging items (Arc of Fiery Ribbons, the Crystal Dart).
As a minor focus, it would be that within one technique and form (e.g. rotting wood, fire attacks, acid attacks).