To find Tonberrys stay near the entranceof the Centra Ruins. You must defeat 18-22 Tonberrys, which will then lure out the Tonberry King. I suggest use Demi as magic and Diablos as a GF. Once the tonberry gets close ranfe to your characters, it will use chefs knife which causes alot of damage. After defeating the last Tonberry, the Tonberry King will appear and you will have another battle. So its important that everyones always healthy.

However during the Tonberry King battle Diablos is no longer useful because the Tonberry King is immune to Diablos attack. I suggest use everyones Guardian Forces. Cast Haste on all your party members and use protect aswell.