The Tonberries attack by slowly advancing towards the party. When they reach the party, they will use their special attack, Knife, to deal massive damage to the member in front of which they are standing, usually killing them. They will then return to the far side of the battle screen. They counter magic with the powerful Holy spell. They have high evasion.
The player should stick to normal attacks to defeat them. A great technique is to cast Invisible on the party. Using only physical attacks can be tedious since the Tonberries have a high evade rate; therefore equipping a Sniper Eye relic to at least one of the characters can help. Special techniques, such as Blitz, Tools and Bushido, will also work well. Slowga is effective for delaying the Tonberries' turns and thus their Knife attacks.
Alternatively, one can Silence them, and use magic (which can include poisoning them with Bio).
Another strategy is simply to Imp them. It hinders their physical attack power and stops them from counter-attacking with Holy.
The easiest strategy to defeat them is to spam Ultima.
Tonberries is one of the four Rages that cannot be learned by Gau. In the case of the other three such monsters (Siegfried, Typhon, and Death Warden) this is because they do not appear on the Veldt, and thus using a cheating or hacking device to make them appear there will allow Gau to learn their Rage normally. This is not true of Tonberries, who do appear on the Veldt. The reason their Rage cannot be learned is different: there are 256 Rages available but because of a bug in the way the game accesses this data, the Tonberries Rage, which is the 256th Rage, does not work properly and was removed from the game. The Rage can be added to Gau's Rage list if he "learns" it on the Veldt, but it will not appear on his list when in battle and thus cannot be selected.