Rosine’s true form looks a lot like the giant monster than Milim defeated a while back. I think the fight will go in much the same way too. Rosine isn’t fast enough to dodge Milim’s attacks and certainly doesn’t have the durability to tank them either. That means that right off the bat she is at a big disadvantage which won’t be easy to overturn. Milim also has greater battle experience. Milim wins.