The Deathless were growing stronger the deeper into Ariel's Hell they went, yes — but Sunny and Nephis were learning how to fight them better, as well.
There were the mundane lessons of how to dismantle the undead warriors faster and more effectively. There were the broader lessons of developing better strategies and implementing better tactics, as well. Most importantly, Sunny and Nephis were learning about the nature of the Deathless, and therefore about their weaknesses.
One of those weaknesses, for example, was the tenuous grasp the undead warriors had on their Aspects and powers. Most Deathless had either lost them entirely or could only use them on a shallow level, as if using muscle memory to do something that they did not understand or remember anymore.
