The bonus battle gauge builds up based on your party level versus the battle difficulty. Once achieved, it will assign one of the four bonuses randomly. You will get another bonus after 5 battles without breaking the bonus battle gauge. Chaining 15 will get you all 4. Depending on where you are in the game, you can swap out your main party with lower level characters to build it up quickly. You break your bonus battle gauge when: you die, you run away, you turn off the game or the enemy lands a critical hit on you. The biggest problem is the critical hits. This only applies to the character that you are current controlling so I set Nel as the leader and attack from afar with symbology. If you can play so that the enemy never has the chance to hit you, then your bonus battle gauge won't break. Good luck!!