There are 2 types of teams:
Physical team: physical defense down + Physical dps 
Magic team: magical defense down + magic dps 
For tanks, they also have physical tanks(miyako, Nozumi) and magical tanks (kuuka, Jun), which are more durable when they endure the respective damage.
You should have at least one debuff unit in your team composition, such as Makoto for physic and S.Kyaru for magic. 1 tank in the front line, and 1 or 2 dps in the back. Sometimes you also need a healer. Read your character skills and you will eventually get it. Basically, you should form a specific team comp only. Don't mix between physic and magic. 
In PVE, you will encounter monsters more durable against physical damage or magic damage. So form your teams wisely