When the second phase started, a huge omnidirectional wave of light would spread and incinerate everything in its path. To get through it, August needed to use his mana precisely to meld into the light's vibrations and phase through it.
This was an intensely involved process. August could do it, but he found that he expended too much mana in the process and wasn't able to sustain himself through the rest of the coming phases.
It wasn't as if he hadn't tried to attack before the second phase before, but the golden shield that the First Dragon Emperor erected was thick and unpierceable by physical or energy-based attacks.
There was only a fraction of a second between when the shield went down and when the light wave appeared. If August couldn't catch the exact timing, he would be turned into ash before he could do anything.
