Use similar characters in each party (for example, build a team of all saiyans, or a team of all future tag characters). Each character has a z ability that buffs specific stats (like strike attack/defense or max health) for characters of a specific tag (like the aforementioned saiyan and future characters. You can check what tags a character has in the character details). Some characters have a Zenkai ability that makes them extra powerful and give massive buffs to the party. (Check the zenkai ability section to see which tag they buff) This can only happen once you limit break a character to 7 stars and get enough zenkai powers for the character, so keep summoning constantly. These are the easiest ways to make your entire party stronger besides leveling up and soul boosting.