DRAGON BALL: Sparking! ZERO

The easiest way to do this in my experience:
1) Main menu
2) Battle Online
3) Player Match
4) Create Room (make it private so you can invite a friend or someone from a TrueAchievements boosting session)
5) Select World Tournament + Under Rule Settings (LB) make sure that "Out-of-Bounds rules" is set to "On"
6) When the battle starts, one of the two players just has to run out of bounds
7) After the fight is over, both players click rematch for the same settings to be applied immediately
8) Repeat this until you have the 100 online battles completed

This process took approximately 60-70 minutes from 0 to 100 battles completed.

This is doable through online tournament with 1 other person and fill the rest with CPUS the CPU battles count towards progression meaning for everyone tournament you do you gain 3% towards the achievement

** MAKE SURE YOU HAVE AT LEAST 1 OTHER PLAYER WITH YOU IN ORDER TO FILL THE REST WITH CPU OTHERWISE IT WONT WORK**

Hope this helps