War in Subterranean World: Avron vs. Hippogriff
There was a reason why Souta was confident with his proposal. He knew that he wouldn't be bound by the Terror Gale Hippogriff. The inheritance of the God of Space was gone so the Red Matter Association acquired it for sure, if not then it would only be one of the locals.
The Red Matter Association was staying in this land for several years and surely they already discovered the existence of the fifth stage monster. They just probably didn't do anything that would arouse its attention and in the dark, they were preparing to eliminate it.
The presence of a fifth stage monster was like a time bomb for the Red Matter Association. They could only sleep peacefully in this land when they were sure that they had the power to eliminate it.
At the very least, no one would escape unscathed if the fifth stage monster and Avron clashed. It doesn't matter who wins among them, the Athen's Champion would eliminate them and claim victory at the end of the day.