As a WHM I have no chance to get gold on Behemoth or Odin due to the credit on boss fates being totally messed up. I just killed Odin yesterday and surprisingly he was up on my screen for most of the fight. I was in cleric stance and spamming stone 2 or holy as much as I could, and only got silver. Same thing happens with Behemoth. I did see some people got the achievement for getting gold so it is possible... How? I do not know.