User blog comment:FairyRaven/Fairy Tail Chapter 391: Gray vs. Silver, Review and Commentary/@comment-37.6.231.28-20140704083655/@comment-7418318-20140704092147

Even though ice magic doesn't hurt him, a fully formed ice-object like an ice sword is still a physical object and should be able to cut Silviora. So maybe it's "just" a matter of delivering such an attack before the Demon has a chance to suck it up.

If there were two of them (cue Lyon) they might pull it off somehow.

And there's still the chance that Silver's soul is imprisoned within the body and might fight Deliora for control over the body. I don't think Silver would be successful, but maybe the internal fight might provide an opening in Deliora's defense long enough for Gray to strike successfully, whether it's with ice weapons or more rocks.