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.
Suggested by iKnowledge Ninian has some pretty good magical abilities like her fire blasts but that won’t be enough to stop Rosine. Rosine has her awakened form which greatly increases all of her stats like speed, power, and overall durability. None of Ninian’s attacks would likely do any damage here and that’s why she will end up being outmatched. At the end of the day Rosine can dodge any attack and deliver a really devastating counter. Rosine wins.