New players, just sort by power and be done with it. (Ironically, that's also pretty much the setup to which you return as a veteran, but in between you may have to make adjustments.) The only thing you really need to decide is which girl you need to upgrade and when. I don't remember how I did it with my first account, but since then I've started two more accounts to experiment...with one I'm concentrating on upgrading just a few girls, mostly in team 1, to let my higher level allies use more of their level-limited strength. The other one I'm upgrading the girls more equally in an attempt to not rely on allies so much.

So far I'm finding that it's pretty much a wash...some maps cater to one approach and some maps cater to the other. Neither method is a shortcut to finishing either an event or a campaign map series. I see this as a very good thing, showing that the game is balanced for rewarding hard work and can't be beaten with any cheap low level min-maxing.