Instructions
Achieve high enough level for your group to properly challenge the Tonberry. Low-level group healing is not very effective in a battle with a Tonberry as attacks are very powerful. It is best to enter the battle with a full Technical Points (TP) bar so that the Renew spell can be used twice to fully heal your entire group. A full TP bar also allows for the use of the Quake spell, to which Tonberries are weak.
Enhance by buffing your group's attack and defense skills with such spells as Fortisol, Aegisol and Deceptisol. This has the added benefit of allowing your group to attack first.
Lower the Tonberry's attack and defense powers (debuff) by casting spells such as Slow, Deprotect, Deshell and Imperil before attempting to attack outright. Tonberries are relatively slow enemies but care should be taken to defeat them before one reaches your party as it has devastating attacks.
Target the Tonberry with Quake, if you have enough TP, then attack with your group's full force. Continue to attack until the Tonberry is defeated.