I'm also wondering... considering the strength of of devil slayer magic.... she shouldn't be able to pull the typical insta-freeze that Gray and Silver does?
Personally I always thought that was more indicative of Silver's power as a mage, not of the magic itself. Everyone tends to think that "oh, if Dragon Slaying was introduced first, then God Slaying, then Devil Slaying, then that must be the order of strength of the magic" but given the typical themes of growing stronger through friendship I'd say it's more dependent on the person.
Romeo with Devil Slaying Magic would still get punked by, say, Gildartz, even if the old man only had something like Levy's Script magic, because Gildartz has so much more raw magic power (more MP) and experience.
That said, Gildartz is probably not the most creative of individuals. So it also depends on how well one uses their magic. But he'd probably just write "PUNCH" and then Romeo would fly to the next continent.