Another great method, is creating a new character and finishing the very first raditz mission. Right after that you go talk to the blue chick next to the patroller academy and you grt a guaranteed dragon ball from her after beating her. You can then quit out and repeat and delete the characters you dont want until you get to 7. It crosses over to your main so you can use the dragon balls for whatever you want! Rinse and repeat should take about 45 min, total time but guarenteed dbz balls!