The Stealth Stinger Spirit was not far from Legendary Quality in its own right.
Consider this: even after nurturing the Giant Brutal Butcher for so long, Fang Mu still had not been able to cross the final threshold into Legendary Quality!
The Stealthy Hidden One's Silver Grade Skill [Doom Scythe] added the ability for the Stealthy Hidden One to battle across tiers.
Even if others wanted to achieve such a breakthrough with their Cultivating Beasts, it would be extremely hard, even with purposeful cultivation.
However, with Fang Mu's beast nurturing, every single one of his Guardian Beasts possessed the ability to fight across tiers.
This, in part, had to do with Fang Mu's Beast Cultivating method.
Fang Mu always provided the top quality resources for his Cultivating Beasts.
Even a holy Grandmaster Creator wouldn't be able to give better resources to the Cultivating Beasts than Fang Mu!