Clash of the Titans [3]
Damien's Nonexistence was well known already. He used it for many years before coming up with a concrete style, and due to the environment that he trained in, he was able to make eons' worth of progress in just a few years.
The Dark God, on the other hand, had eons worth of practice but no solid environment to support his growth. It was for this reason that they were equal in terms of power.
The disparity came from the fact that the Dark God had never faced another Supreme. He had seen the Worldwalker before, but he'd never seen the Worldwalker fight. In the same way, because Supremes were so rare, none of the cosmos he'd conquered until this point were able to birth one to face him.