The typical RPG group consists of a tank, two characters focused on dealing damage, and a healer. While that setup works well in this game, you do have some flexibility. Since item use doesn’t use up a character’s action, healing with items is limited only by your stash of credits. Use the advantages of front- or back-line placement, and don’t forget you can swap rows as an action.
As your characters gain levels and can purchase new abilities, always choose complementary abilities. If something works better when a certain status is in effect (such as Burn or Poison), then choose abilities for other characters that inflict that status effect. Only four abilities can be active at a time.