Had to figure this out for ourselves after spending nearly 40 minutes on the final boss.

If we used Holy on the final boss, then the Tonberries became unkillable and kept chasing and stabbing at zero health. Not sure if other classes cause the same effect. We had a PLD, WHM, MNK, and BLM. Tonberries only became ZombieTonBerries after using Holy.

Kinda Confirmation here: http://ns.reddit.com/r/ffxiv/comment...rs_palace_psa/

My apologies if this has already been posted, I couldn't find it.