Originally Posted by naenae

Is it more advisable to try to raise one girl at a time or should I level them as a whole? I have all the units I use at about level 30 right now.

It's generally advisable to keep the girls you have in your first squad about 20 levels ahead of the rest of the girls you use in the other squads.
Two reasons for this:
1. That first squad is the "helper" squad for your allies, so they'll appreciate it if you try to make it as good as you can.
2. Helper squads you borrow from your allies (and other people) get tuned down to the level of the highest level girl on your own team. So if your highest level girl is lvl 30, and you use a strong helper squad from an ally with girls of a higher level, then those girls will only perform as lvl 30 girls as well.

Also, make sure to color match the manyus with the little icon in the top left of your girls' shield. So purple manyus go to girls with a purple icon (magic), yellow manyus to girls with a yellow icon (pierce), etc. This is because matching those colors will give a 50% experience bonus, so you'd need only 2/3rd the amount of manyus (and gold!) to max a girl that you would need without color matching.