How can there be a Nun movie?
She said it herself, "Your name gives me dominion over you demon!"
So it's pretty easy to defeat Valak. All she has to do is cast it out.
Well, it did take her a while to figure out the name. So, without that bit of information it could be a pretty threatening presence. Not sure if that premise can be stretched out to a whole movie, but we'll find out soon enough I suppose.
https://www.youtube.com/watch?v=uiog10FEGA4