If you really want to take a crack at the Master Tonberries, have someone in your party who hasn't made a lot of kills. Make sure that character has some sort of attack, skill, etc. that can kill the tonberry within the four waddles that he takes before attacking and you're all set. Having someone that has a low kill count, attack Master Tonberry, will greatly reduce the amount of damage he does with his counterattack, Karma.