Every hit was disrupting the closed environment of the two realms. Uriel was trying her best to analyze and keep up. Not only is she doing her best to keep the Shadow Space: Unending Darkness intact, but she is also dealing with containing the strong impactful waves of two negating energy.
Although Seth said that he'd have the upper hand in their clash of sword and scythe, he was clearly outmatched.
The experience and training were enough to easily outclass him. Add to the fact that Seth had no training using this new form made of pure shadow from the Shadow Realm, it was a matter of time before he got worn down.
Uriel was actually not fully using her Complete Analysis on the Domains but just a majority of it. Seth was so outmatched that Uriel kept on reviving multiple Dark Shadow Legions made as a vessel.
