Been playing since release, I now have all the girls from all the boss, only one I got from pachinko, because I gambled I wouldn't get her and get an event one instead(bad idea).

Rest were all drops from the boss, never paid a dime.

The last boss(plains of despair) had a 100% drop rate on all 3 girls on me, not sure if that's a bug or not. For the rest you just need to keep hitting over and over. Most took weeks to get the last girl from them, but eventually you hit that 1%(arbitrary number, but feels right) chance and get them.

I also have a mouse macro to auto hit boss while I sleep mind(does the fight every 20 mins, time to get 2 stamina).
Most of the time I end up getting them while I'm awake though.

Always get both girls from every events too, usually take a little less then a week per girl(and each event is 2 weeks).