Well, it's been a while since I've played, so I will have to post about some boss drops from awhile back to gather more info about how much the difficulty raises on boss drop girls depending on the rarity.
Event, 36, 17, Dark Lord(lvl 40), about 180 battles, Gwenalle
Event, 43, 20, Ninja Spy (around 20?), about 100 battles, Oxton
I am currently trying to get Golden Bunny from donnatien. I have tried 14 times up to now, with one x10 perform. He was lvl 56 when I started, with him getting to 61 by now. As I said, I haven't played in like two months, with Beach Eugenie being the last girl to win back then and the last girl on my harem now, with a total of 25 girls. Golden Bunny is a Legendary class boss drop, so I suppose 'll need at least 200 battles, if I'm unlucky.