i play the empire most of the time, and (if i'm attacking) i found that a good strategy is to bring 1-2 squad TIE mauler squads, 4-5 trooper squads for low end battles 7-9 for tougher battles, at least one AT-ST squad for easy battle, and at least three for the harder battles. all that coupled with, at least one hero, (Vader is prefact)
i can win most battles no sweat.
i like using vader on tough anti-infantry towers, cuz he can disable them well my solders move up and blow the S**T out of it! XD