I've personally never even been attacked by a to berry in that fight, so I can't say anything from experience.

However, my answer to your question is based off of the situation at the moment. I have three answers.

1. If the tank has full/near full Hp, and mp is not an issue, AND you don't have any AoE, don't be afraid to Swiftcast/Holy a bit. However, this may cause more tonberries to attack you, leading to a slippery slope.

2. If the tank has full/near full Hp, and mp IS an issue OR you have good AoE, run into the center stack and have the the AoE to kill it and keep up with the tank.

3. If the tank has less than half Hp (around 1/4 left really), it's up to the Dps to identify that and take care of the Tonberry OR if you have good AoE, run into the center stack.

At least, that's my opinion on that.