from the core book under sample major magical foci... "Damage: Any Art, as long as the effect does damage directly, either by inflicting a wound or doing a certain amount of damage which can be soaked."
Does this mean pick one art and any way you use it for damage applies? Or does it mean "ANY" art that is used for damage?
It means it applies to any/every Art combination that inflicts damage.
Although that sounds super-powerful, I would say it's not one of the more powerful major choices. You can pretty much get the same effect out of a minor focus unless you want to be a partial generalist (such as a Creo specialist or Perdo specialist with a mix of Forms). The reason for that is that foci are only particularly advantageous if you raise the relevant Arts noticeably. Otherwise you could just put two of the three points into Puissant Arts to get +6 and save yourself a point. As a result you will probably find most damage-dealing magi will tend to use a single TeFo combination heavily and can thus benefit as much from a minor focus while saving two points. However, this focus can be particularly useful. For example, let's say you have a Flambeau Creo specialist who wants to mix up CrAu lightning, CrIg fire, and maybe some other CrFo attacks. Such a focus could work very well in that case. The same would be true of a Perdo specialist who picks up some PeAn, PeCo, and other spells like those.
Chris
That's about the size of it. And I'd widen it to include causing objects to make Stress Checks as per City & Guild. One thing I'm not sure on, and we don't use in my saga, is whether Might stripping counts as "damage". We've ruled not as we think to do otherwise would broaden the Virtue beyond even it's already quite broad definition.
I agree with Mark about the Perdo/Vim question. That's not really damage in that sense.
Damage , is normally physical damage , caused to an animal , plant , object or person.
If you are affecting a Ghost or some other non-physical being , then Might-stripping probably should count as damage.
That would be from the actual Might Score , not the Might Pool used for various powers.
I think the best chance to find a specific comment on might stripping is in the section about healing stripped might. One of the books has a page with a comment about spirits healing stripped might, in one day if memory serves as long as they haven't been destroyed. But I haven't been able to find that page in ages. 
Anyone know where it is?
Chris