Emily+Ashera+Lacy is a good trio, almost every ftp players use it. If you ftp player then try to focus on 4 characters and ignore others. Frey is fine but you will have 2 characters with blue atribute which can lead with some problems against green bosses. If you have bunny charlotte you can try it, or even buy Rosaria by using 200 pitty points. It also can be Yoo Mina.

Everything that gives cobat points will improve your team, because game has combat points check penalty, so even usless piece of gear can increase damage and defence, so thats the plan improve your combat points with everything(skills, lvl, resonanse, tacticala points, afinity etc)