Between Zach and the Child of the Hydra, Zach's only advantage was the power of the world essence energy in his blows. Usually, he would have also had the restorative effect of the world essence energy on his side. But the scars from last time were still hampering that, putting him just a margin above the Child.
In terms of sheer strength, raw firepower, sturdiness, and speed, the Child surpassed Zach.
The injury to his knee lowered his speed to just above Zach's, and Zach's barriers made up for his lacking durability. But there wasn't much he could do in the other areas other than try to explode the Child with his energy, which wasn't going very well since he was far from skilled enough at handling it.
However, the skull was one of the hardest pieces of bone in the human body. Although what was inside it wasn't very durable, as long as the skull itself held up, it would be mostly fine. That's what Zach hoped at least.
