Bards or Black Mages.

Bards
They love to pull the initial aggro and then proceed to fire off continuous shots until it dies. Then the poor tank is trying his hardest to get aggro back and to keep it after he gains it. Meanwhile, in the phase in between the pull and tank recovering it... the healer obviously pulls the un-held aggro the Bard pulled. Just one giant mess of a situation.

Black Mages
AoE. That is all. They spam Blizzard II over, and over, and over and over. Problems? A) Makes everyone lag a bit more. B) Steals MASSIVE amounts of aggro. C) Doesn't do nearly as much damage as they think it does. Solutions? Move the AoEs up to mid 30s. Then you will atleast have experience when you get it.