"Final Fantasy X" features dozens of rare enemies that you'll encounter during your quests. Master Tonberry is an unimposing figure who will creep steadily towards you without attacking during a battle. If he has enough time to take four steps towards you, he'll perform an attack that will kill your entire party. Two methods exist to make sure that this doesn't happen, and they involve either using conventional attacks while Master Tonberry is sedated or one massive attack to finish him off instantly.
Go to the Omega Ruins and begin a fight with Master Tonberry. He is a random encounter, so you may have to fight or flee from several unrelated enemies before you see him. The stats for your characters should be at or above 120.
Cast "Hastega" on your team to speed up its attacks. Cast "Sleep Buster" on Master Tonberry to sedate him. He may resist this spell, in which case you should keep casting it with other team members until it is effective. Putting him to sleep will nullify his "Karma" attack, which causes him to strike back at any member that physically hits Master Tonberry. Keep casting "Sleep Buster" on him and "Hastega" on your team while your heavy hitters attack him physically until he is defeated.
Summon the Magus Sisters. Only Yuma will be able to do this. Use the Overdrive attack of the Magus Sisters, which is called "Delta Attack." You'll need a full Overdrive bar for the Magus Sisters to use this attack. This Overdrive attack is available immediately after you get the Sisters, or their Overdrive bars will be filled slightly each time they take physical damage. The Delta Attack will either kill Master Tonberry in one shot or he'll be so damaged that he can be finished off with a few physical attacks.