Steel beats ice didn't you know?
We don't even know how strong ice shell is. Just because it was able to contain some featless monster "forever" doesn't mean it can contain someone as powerful as Gajeel.
really you think gajeel could possibly be stronger than deliora? iced shell is the definitive ice magic there is nothing gajeel has that's even close.
deliora's casual attacks do this:
deliora is one of the strongest demons of zeref, we just saw another etherious go toe to toe with the friggin spirit king. they are up there with the strongest fairy tail characters.
iced shell has a stopping power similar to that of fairy law, possibly even above.