In order to make a stronger party, you must grow your units and form a party strategically.

Go to the Units menu at the bottom of the screen and select Raise Level to use the EXP acquired from rewards to raise the level of a unit.

Also, if you own dupes of a unit or Astral Gems that correspond to a unit in rarity, you can raise this unit's maximum level from the Uncap menu.

Go to Units at the bottom of the screen and select Learn Ability to enter the Mana Board to strengthen a unit's options by using material items and Mana.

Keeping in mind each unit's leader talent and abilities, and the power of different armaments will help you form a party in a more effective way.