Go all in with a multi-character attack that fight all at once it awards you with the appropriate boosts/perks needed to strengthen your squad.
don't forget to get you free cards every day. Any card you don't want can be used in the evolving and transfusion process for your more powerful cards.

If you start to run out of room in your inventory use the transfusion to clear space and to power up the main members of your squad. The rarest members should be the ones to get the power ups, because they are the least likely to be replaced.

Use Delivery Box to help store any items you don't have enough room to hold. This box can only hold up to 200 items and for up to 14 days.
Completing missions depletes your stamina, but the rewards you're granted by finishing off missions always pay off in the end.

Use up as much stamina as possible to finish off any missions. Doing this guarantees that you'll have a full stamina bar when you exit and re-enter the game on your next playthrough.

Select the Auto-Build action if you're looking to go into battle with the strongest cards possible. It saves your time, but it also keeps your best cards on hand at any moment in battle.