I've been playing for almost a year and a half now. I still have 5 boss girls to get. 2 of them from Hamelin Town and all 3 from Magic Forest. I've maxed both of the bosses' levels, but have just gotten rotten RNG. I stop during events to try for event girls and if I need a drop to advance the story from sensei, but other than that I just keep going back to those 2 to try and finish up getting the boss girls. I do have all the bosses at their max level though, so that's a good thing.