User blog comment:FairyRaven/Fairy Tail Chapter 390: The Story of a Boy, Review and Commentary/@comment-7418318-20140627093953/@comment-24066070-20140628164321

@Nick: The Ice Magic itself may have sufficient potential to kill even a Demon created by Zeref.

It's possible that Ur's magic at the time wasn't strong enough to sufficiently damage or kill an Etherias (I think it's safe to assume the actual member of the Nine Demon Gates is Deliora and not Silver, it would also justify Deliora's power as it was shown in flashbacks, which was far greater than anything shown by the current Etherias.), so Ur had to resort to her strongest move, Iced Shell, to do the job.

Deliora on the other hand likely has greater power than Ur at the time, so he can exert more power compared to Ur, thus resulting in the stronger ice present at the Sun Village.

Furthermore, Doriarte is without a doubt weaker than Deliora, thereby requiring less drastic measures like Iced Shell.

Bottom line is, Ur was weaker than Deliora, so she had to resort to sacrificing her own life in order to take him down as well.

Whereas Deliora is far stronger than Doriarte, so just the residual magic in his ice is enough to defeat Doriarte.