Keep a healer on your team, especially for Story missions. Although they are not as effective in combat, you can Train and Promote them in order to make them stronger.
Training
Save your Credits for training. Do not use them to purchase Hero Crates after your team is effective. Save at least one of each Promotion Item and use it when you begin getting your team up to their maximum Tier levels. Subsequent Training sessions get expensive. If possible, use as many Training Items (up to five) as needed every time.