Layout
When starting a mission, adjust your party layout. Place a ranged character in the middle with Groot nearby to have the ability to heal everybody. Place tanks or melee units to the sides. Different characters work better when paired with a matching person. For example, Drax and Gamora, Groot and Rocket, etc. Team them up so that you can combine their power-ups for increased effect. Keeping a safe distance between characters to minimize crowding and allow you to easily select one. When a character dies, even if the battle is won, they will not gain experience. Try to keep all of your characters safe. When their health gets low, move them away from battle and allow them to heal.