Otis began foaming at the mouth because of the severe pain. He exerted himself fully, growing as many mushrooms as he could. Unfortunately, just a few seconds after any mushroom formed, they burned to ash.
Apollo had a relatively clear path, but if he recklessly shot forward some of the fire might catch onto him. Although Apollo didn't understand how the flame of a phoenix worked, it didn't take a genius to realize getting touched by it was probably a bad thing.
Otis realized the futility of his actions and fully disconnected his senses from the mushrooms. Unfortunately, the pain didn't disappear, just like last time.
The mycelium and mushrooms stopped growing.
As such, what remained quickly burned to ashes and began falling to the ground. Since Otis depended on the mycelium to remain so high up in the air, he ended up falling through the sky.
However, that was the least of his concerns at this point.
