By having characters maybe foreshadow things instead of literally explain them. Why did we hear absolutely nothing about any of the things I listed before we were given either an exposition dump or just sudden apperence.
I mean with the Dragon, maybe it has something to do with the Vale outer city that failed. It seemed to come from that direction. Maybe Oobleck could have mentioned something about a larger Grimm being seen before the city was sealed, or perhaps anything.
The Maidens really were an asspull. We've never heard of them, seen them or any methos surronding them, and SUDDENLY they hold unimaginable power and that power decides the fate of humanity. Literally Blake being a little bitch has more backstory then the deus ex machina of the whole series. That's just poor writing, or more likely the result of poor pacing.