If you're stuck at some campaign stages: that is actually quite normal, even when you do level your girls optimally. I can't remember where it was exactly, but somewhere around the 9th mission, the difficulty ramps up very quickly, meaning every player gets stuck around there for a while, especially without powerful helper squads. Mission 12 is another one where I got stuck on 12-4 and 12-5 for a while. Gladly, 12-3 is a great map to farm for gold and also some manyus on the side, so it was just a matter of patience grinding that out to improve my favourite girls, replace several 2*-4* girls with 5*, until I could reliably clear 12-5 by myself, so even when coupled with a weaker helper.