Familiarize with each character skills and their cooldown time, which could give you a good clue as when to use a particular skill.
Gather Data Cards by completing special events to unlock characters.
The more events and missions you play across the various difficulty settings, the better your chance of building a solid roster.
Watch the blue meters positioned under a given character's health bar and when that bar fills, the character gets to attack.
Target one enemy at a time, so that you can more quickly get rid of sources of potential damage.