Hm... This is obviously speculation, but I've rewatched the trailer and noticed a few things I hadn't before. This girl in red seems to be the one who summons "Aversa". She seems to be doing it in the city, under open skies.
The end of the trailer shows the blue haired girl confronting Aversa in the city too. However, her in game battle scene shown earlier in the video is in that stage-like place seen in all gameplay battles in the video.
I wonder if every battle actually takes place in those environments automatically, doesn't matter where they are in the story? There are a few variations of those stages shown in the trailer too, like one that seems to have an ancient theme to it.
The ending of the video, when the blue haired girl confronts Aversa, seems to show many of those shinning circles, similar to the one bellow Aversa while she appears, all around the city.
If those circles are bringing more monsters like Aversa, could that be what leads to this?