In general, once airborne every dragon's weakness seems to be the wings, vulnerable to severing and disabling by other dragons; and in GoT we saw what a well-aimed ballistic weapon will do more than once.
They're devastating to unprepared ground troops and fortifications but rely a lot on the element of surprise and general air superiority via tactics (cloud cover, topography, sun etc.) and likely superior numbers; hence my guess that two smaller dragons that can close on Vhagar from above or behind will make short work of her.
They just have to keep her head away from them via a third, likely sacrificial opponent or by holding her off long enough to disable one or both wings.
