Just to add something that I haven't seen mentioned:
When I was first starting out, not too long ago (Geranium's event), I found it to be very useful to create teams of girls with similar survivability, that is with a close balance of Health and Defense. The idea being that if half your team is easily picked off, you end up in a situation where your strong girls are taking a lot more hits and therefore become less effective overall. My first draw was DiYu and she has very high defensive stats, so she would often end up fighting alone for several rounds.

I still try to balance my teams this way, but I find that defense seems to have less of an effect now as enemies that are strong enough to down my girls do so in very few hits. Additionally, this effect can also be achieved by grouping girls with similar survivability together, but it is still a major consideration for me when picking equipment for a 1 or 2 slot girl.