Did you evolve your girls and level them up again ? That pretty much doubles all of their stats (Units - tab number 4)
Did you spend gifts to raise their affection to 100% ? Also gives bunch of additional stats
Did you give them equipment ?
Did you play the actual tutorial that explains bunch of this crap ?

All of these questions need to be answered by yourself as those answers are most likely what you are looking for.

And yes eventually there are maps with difficulty high enough to 3 shot a maxed out, ampy boosted 6*, however those are not in this version of the game, and will not be implemented for a VERY looong time still.