...Grigori's Large Country level is kinda undefined, we just know that's his lowest-end.
Err, it's good enough to create alternate realities like Bitterblack Isle, for starters, though I imagine that unless we were using 3-B Grigori he'd not be able to create anything quite of that magnitude. Other than that, there's assorted just messing with the world he pulls off, including-but-not-limited-to: sinking entire cities, opening up portals to the Everfall, dramatically changing what flora and fauna can be found in the world, warping the skies to a shade of green, being the direct source of all of the other Arisen's power and immortality that accepted his "deal", ability to control the Will/Souls of others and via doing so awaken an Arisen's latent potential, and straight up immunity to a variety of ailments, including Mind Manip., Petrification, etc. Curiously, he's not immune to the instant-death effect Maker's Finger, and I've not been able to test whether Exequy works on him or not, but it seems as though he has, if not a vulnerability, then no special resistance against OHK techniques.
What sort of Darkness Manip. does Grima have, btw? It doesn't really elaborate on his profile, his only Notable Technique listed is "Breath." Also, does Grima also have Type 2 Immortality? The whole "only the Chosen One can kill me for good" deal as Grigori, in this case an aspect of itself and/or Falchion? I guess that's probably also a problem for Grigori to deal with.