I assumed this was the spellcasting animation, but in this case, there is no cast time, and you become temporarily invincible.
Hmm, interesting idea. Maybe it works like FFX or XII, where the summoner disappears and controls the summon directly. Except instead of disappearing, she hides behind a shield.
It looks pretty big for that, though -- it definitely seems to be sized for a party, and there's another guy in there with her.
Which leads me to believe, maybe just maybe, leviathan was actually a summon CS, like the old style FFs.
Oh god, if summon cutscenes were annoying in VIII I don't even want to think about watching them every couple minutes for 4 years.