I started playing this right when the Moonvine event begun, that's around 52 days ago and i upgraded the 2* girls, i already have maxed out all of them and i can say the investment was worthy, they allow me to clear Ultimate maps, i can't say with ease but with some planning and strategy i have cleared many Ultimate maps with 3 stars

I'm currently working on the Copper units and i'm planning to max out all of them too, i will recomend to lv up your girls equally, if you concentrate all your efforts on a few of them you will eventually run into a situation where the enemies are to many and stronger than your squads, it's better to have squads with all the members at lv 40+ than a squad with 1 single unit of lv 50+ and the rest with lv 20-30, also try to keep all your squads with similar speeds, when you need one or two speedy squads you can switch girls form squad to squad to get the speed you need for a certain map

Here's my Squads:

I keep Di Yu because she was my first unit and had max affection from the start and those affection bonuses helps her a lot even when she is just lv 31, she will stay in the squad until i max out another Copper girl, i'm not saying Silver or Gold girls aren't that great, they are awesome, specially with their powerfull skills which gives stats boost, but if you don't have to many of them you shouldn't be afraid to invest on the lower rarity girls, they can fight too