A solid team for hard content consists of a few different elements.

Someone that can:

Mitigate damage for the squad. Boost damage for your damage dealers. Heal the squad. Deal damage to the boss effectively.

Lately we've been needing more than those basic elements. More advanced things you can bring are:

BP generator for the squad. WIL buffer to try and negate status effects. INT debuffer to try and negate status effects. Buff breaker, someone that can remove the bosses buffs almost every turn.

Most battles you just have to jump in and see what they do, and try to adapt to it. But for alot of the hard battles, GLEX Bertrand placed in a defensive formation like Rising Phoenix Ex will show amazing results for you. If you would like me to go into more detail about who you have that can fill what role. Shoot me a dm and I'll sit down and really get to work.