This is why I wrote this some posts before: "When there are no more girls, the boss will drop an affection boosting item, which will raise it by the same value of the boss level." It may seems too much, but 20 battles on a 100 level boss would mean an additional 2000 in the affection of a single girl; it is not an hig value. Of courser there are stronger bosses, but there are also girl with more than 10000 points required for one level.
Plus there is a new 5 star girl coming, and we all know how long it will take to make her out, provided that you get it first...