This is not about counter-spelling.

Lets say there is a magus/monster/blah whatever infront of me that is normally humanoid in form. This antagonist, decides to shapechange/shift/whatever into an earth elemental/living flame/anything totally alien.

As a master of corpus, I mouth "oh crap" as I am no longer able to affect this individual with a Corpus spell. However, I do have an unraveling the fabric form (Corpus), I figure I can cast it on this person (since target is individual and transformations are almost always MuCo spells)...

So I cast it. It goes through their magic resistance.. and .. I .. uhm... hmmm

The I what? I test to see if I can dispel a single corpus spell that is on my target? Do I test for all spells? Just the high level one?

Basically, how would you use Unraveling the fabric form as a dispel? And can you?

You can find information on which kind of body transformation allows you to use which forms in the Bjornaer chapter of HoH:MC.

Since the persons essential nature is human, can't she always be targeted by corpus magic? (Heartbeast aside that is),

The errata entry for p117 certainly implies that a human shifted into an animal is affected by corpus and mentem magic rather than animal (again, heartbeast aside) - surely the same would apply to a human shifted into any other form,

Of course, if shifted into something totally alien then some corpus spells might struggle to affect you (kiss of death might not work if the new form has no heart) - but it would still be corpus to affect the shifter directly,

I would play it as "I target the transformation" ("cancel the effects of any one spell of the specified form", so the target is clearly a specified spell). Then check unraveling spell level + 10 + stress-die-no-botch vs. the transformation spell level.

Naturally, if the transformation is non-hermetic, heartbeast, or have no corpus component (it's really a MuIm, for example) the unraveling won't work. Unraveling the Fabric of Corpus is really, really specifically dispelling of Hermetic Corpus magic, nothing else. So no dice vs Fae shapeshifting, for example.

The Bjornaer chapter of HoH:MC is recommended reading, in any case.