The best way to go about creating a Grand Sorcery for the Nine Forces of Nature is to figure out what you would want to do with it, which would tell you how best to proceed. In this case of dealing with the Saa, an example would be Stone would seal it away, Flame or Tempest could be used to blast it directly, etc.
The original plan that was suggested of making a big creature from Anima or Verdure would not work. Summoning creatures is not natural or normal, and would in fact be a perversion of Anima to make a creature that is normally not large large.
We could have gone with Flame or Tempest to blast it directly, but this would almost certainly be less effective as the Saa would still have something to focus on to fight. Verdure is the clear choice here as it cannot fight the land itself.
Using Verdure to bind it further will not cause it's defenses to be raised and thus it would be the best to distract it enough to let us strike a fatal blow.
Antairia's Grand Sorcery "Stone's Eternal Decree" would work against it but it doesn't really align with the goal so it should not be used here.
This would be a specific Grand Sorcery of Verdure and not a "basic" one that is commonly available.
If one wanted to use As Above So Below to consult the moons of the Vale about this, Crimson Spark would be best for the battle, and Argent Radiance for the Grand Sorcery.
Lady Ember has graciously found herself with time to research this Opportunity for us, and I believe that Gloamfeather will be casting it should all go well, but I have sent a letter to some non Beacon Sylphs just in case.
Some other digressive notes unrelated to the Saa fight:
A Sun Grand Sorcery would be about protection, and couldn't be used to sunder connections between two entities in this matter.
There is no "unflavored" Grand Sorcery of NFN, other than potentially making an area much like the Vale.
Antairia's Peace manifests near Beacon based on need. For example it showed up when we were fighting the snake goddess.
Antairia's Peace ambiently acquires magical things, which is why Runecrafting material shows up within it so much. Interestingly you rarely find Shadow materials there.
There is no obvious relationship between Runecrafting materials and the Nine Forces of Nature. The Runecrafting materials are based on the six elements of reality, but there are multiple ways of perceiving reality. Be it these 6 elements, or the Five Elements of the Golden Lotus Provinces, or the Nine Forces of Nature, there isn't a "wrong" structure to any of these.
Grand Sorcery of the Nine Forces of Nature to Fight the Saa
Grand Sorcery of the Nine Forces of Nature to Fight the Saa

Or just Thorn is ok
Tempest Sylph of the Vale of Dusk