It doesn't matter what rank you are, you just need 300 or more virtual comments. You can easily get that many by doing a bunch of different things in the battle. I've gotten the items on C-ranked missions. But this means that if you get a rank S by being ridiculously cheap you won't get the item. Even worse, you'll have to get a higher score AND more comments next time to overwrite the existing movie. Worrying about both these things at the same time in a difficult battle is not what you want to do, so don't be cheap. I've never seen a game deal with cheapness in quite so evil a way... A lot of people fin themselves locked out of Blackrose's items.