When Eris noticed that Arad wasn't going to just die from a simple stab or a solar flare, she decided to go back into wasting energy and throw entire stars at him. As long as she had Amaterasu supplying her with divine magic, she should be able to keep it up.
If she wanted to win, she had to put everything on the line.
The problem with that strategy was that with Arad actively trying to suppress her with his authority over her blood, controlling such a vast amount of power was harder than she could safely manage. An entire star, while it looked easy for her to spawn and hurl away, was, in fact, a deadly, delicate balance.
Eris was already punching far above her weight class, so going further was stepping into a dangerous and deadly territory.
