You get 2 Keys a day, but can only have 5 in possession.

I guess a high popularity means they fight better cause they love ya.

As far as strategy goes sounds as if it will be better with higher popularity. But I have had some luck combining Strategies.
It did not cure the the the problem but it did slow down the damage rate.

example 1: HG 1 "attack from south" HG 2 "Accompany short range" now one is not attacked till brake 2 and then the other. the attacks are split between the two and are missed more often

example 2: SMG 1 and 2 "Attack from north" It takes a little longer till first attack but are in position to fire on multiple targets on first attack

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when using up girls in training, does it help to have the disappearing girl(s) leveled?