They can't stay close together or they will risk Aeternus taking out more of them with a single shot. So the remaining Celestials stayed in a loose mobile formation with a wide distance between each of them. They were able to stop themselves from panicking and reforming their forces just in time for Aeternus to collide with them.
The Celestials had scattered earlier when the beam hit their formation. They ran for their lives from the black beam of death. That made them stop paying attention to their attacks. Aeternus kept them busy with escaping with their lives while he expanded the pathway created by the beam. He used his normal attacks to destroy the obstacles in his way and quickly reached the Celestials.