User blog comment:FairyRaven/Fairy Tail Chapter 390: The Story of a Boy, Review and Commentary/@comment-7418318-20140627093953/@comment-7418318-20140628161209

Nice theory Woest, but with one flaw. Gray used Silviora's (Deliora in Silver's body. I think I'll keep using this from now on) against Doriate and it was so effective because it had special Demon Slaying properties.

If Ul's magic had had those properties too, she would have been able to defeat Deliora without having to resort to Iced Shell. And the magic Silviora uses can't be like Iced Shell itself, because Iced Shell breaks the casters body down and converts it into ice. Even if he is able to use only part of his body as "fuel" and regenerate it later, I doubt this regeneration would be instant, but we didn't see any parts of Silviora disappear when he cast spells.