Titus Crow managing to figure out the entirety of the Cthulhu Cycle Deities existence as well as the Chthonians plot against the world on his own. Notably, the Wilmarth Foundation had done the same but they were a massive Ancient Tradition working to that goal while Titus was just a bored bystander with a penchant for the occult.
Titus freeing Henri from the Chthonians' control with an exorcism from an old book. Doubly so for the fact he had no idea it would work.
The Wilmarth Foundation as a whole gets one as they manage to successfully fight a full-scale global war against the Chthonians in secret and win. No, they don't manage to kill one of the Great Old Ones and suffer terrible losses but they prevent the Chthonians from taking over the world, possibly even condemning them to extinction if the few that escaped with Shudde M'ell were all male.
Spawn of the Winds
The fact Armandra has been fighting a guerilla war successfully against her father for years despite the fact he's a Physical God and she's a Black Magician Girl.
The fact Hank and his companions are able to use the collection of Star Stones they've found to win against Ithaqua's armies despite being vastly outnumbered as well as betrayed by one of their own.
The Moons of Borea
After having been chased across three planetoids, the heroes finally get the Clock of Dreams back and immediately turn its Wave Motion Gun-esque laser on Ithaqua. It is not the first time a Great Old One has been defeated by the protagonists but it's one of the most satisfying.