Its an odd proogression path, but you need to complete the "glowing orb/?????" quest that stems from "the hub" She Is a pain in the ass to get,(also don't sell the orb, if you do... I don't know what happens, I didn't sell the orb.)
Some things that help with that quest.
The quest says that you just need to fight robots, and that 9, 10, 12 have robots.
You only need to farm out floor 9 to get the robot parts, just keep mashing floor 9.
my general rule of thumb when ordering teams, High hp low damage up front, low hp high damage in back. reasoning is tanks keep the rest of the team alive by blocking bullets, while DPS widdles away at the oposite team.
You also need to progress enough with the girls to have most of everyone on your team, (I just spammed the gatcha thing and ignored the mats as they do nothing aside from unlocking the girls to battle) Also the entire party I had was lvl 15, no idea if they can get more levels, but thats when they stopped getting levels for me.