This few tips might help you to earn Girl’s paradise achievement.
Tips to Obtain Girl’s Paradise Achievement
You’ll have to max Mona and Nina (10 hearts and have seen all 3 events for both of them) and either one of them needs to have the highest affection from all your friends by the time that you finish the game. It also doesn’t matter if there are more characters that fulfill the marriage conditions as it will automatically choose the person that you have the highest affection with.
In my case, I’ve fulfilled the marriage conditions for Nina, Mona and Gwyneth and I still ended up with the Nina + Mona special romance ending as Nina was my number one.
Just be careful that in case that you want to marry only Nina OR Mona, that you’ll need to make sure that the other person doesn’t fulfill the marriage conditions, because otherwise you’ll end up with the special ending again.