Dragon Ball: Raging Blast

There's an extremely fast and rather easy way:
Once you've unlocked Extreme Battle Mode, you can select a character to defeat Chao-Zu, your first opponent. After you defeated him, a Dragon Ball Fight will be unlocked, go to this fight and win to get a Dragon Ball. Then, choose another character for Extreme Battle Mode and win against Chao-Zu again, then another Dragon Ball Fight will be unlocked, just win this again and you'll get another Dragon Ball.
For the Dragon Ball Fights you can use the same character over and over, just make sure to pick another one in Extreme Battle Mode everytime you fight against Chao-Zu.
You can repeat this until you have all 7, then you've to summon Shenron and select your item to lose your Dragon Balls.
After this, you can start defeating Chao-Zu again until you get another 7 Dragon Balls.

Collect all 7 Dragon Balls in Super Battle Trial. Each time you complete a mode in Super Battle Trial, you unlock DragonBall battle, where you have to fight a set character to earn yourself a Dragonball. I haven't lost a fight while obtaining them so i'm still unsure as to whether or not the battle mode stays open until you beat the specific person or not. fter you defeat the opponent, the mode dissapears until you clear another Battle trial.