Most likely, the enemies used that time to think of some strategies to deal with Billy once he attacked them. Still, he couldn't just watch them keep doing that… large attacks like those not only changes the battlefield too much, but they also damage the barrier and shows more of it that can be attacked…
In any case, Billy prepared to fly toward them as fast as he could when he saw all of the enemies that could fly, making a line to stop him. While it would be minimal, all of them would make the power of Billy's attack decrease, and if he fails at killing them, he will need some extra time to do all that.
"Let's see… I guess I can do that," Billy thought.
Billy approached the barrier normally, and all the enemies raised their guard. Still, they couldn't help but frown when they saw him assuming a throwing stance… using his weapon like that was rather stupid since it was one of the few in the world that could withstand his power.
