Tips and Tricks:
Don't spend Souls on stamina refills. It's better to use them to unlocking Gold characters.
Once a character uses a special move, be sure to tag him/her out so that he can recover and you'll be able to use another character's special move.
As for team tactics start with a strong Silver team with members of the Spec Ops faction or Martial Artists, if possible then upgrade it with Gold characters.
Match your team members so they can work well together with complementary passive skills, special moves, and defensive capabilities.
You can always play same levels for experience and upgrades.