It had been a while since Cain had recovered a decent amount of Ki, stamina, and HP. However, he chose to continue waiting...
The fight between Stingè and the Aether Beast had progressed to a level where he couldn't possibly see what the outcome was. The two Tier 1 beings were moving so fast, he was having difficulty telling what was going on.
The small limbs attached to the creature's sides made it so that the creature didn't just run very fast, it also could respond to attacks aimed at its blind spots quickly.
That wasn't to say Stingè was losing badly. At the end of the day, because Stingè was smaller, he had better maneuverability. He could attack the Aether beast from anywhere.
As the battle progressed, it turned more and more bloody. Cain quickly saw that his initial verdict was wrong.