Strike your opponent 10 times in a Duel
The max amount of bubbles can only be set to 7, so start by doing this. Attack the opponent 5 or 6 times. You will then need to use the other controller to attack the main rocket 3 times in order to get back some bubbles. Go back to the main rocket, which should not have 4 or 5 bubbles and take it out. Once you win the match, it should have been 10 strikes in total and you will get the trophy.