It seems like a lot of people are having this issue in huge fates with Behemoth and Odin and the bosses not rendering. This has happened to PS3 and PC users. I don't know what needs to be done to fix this issue but if the fates of behemoth, odin and others are meant to be an important part of the game, these issues need to be addressed. I apologize if this issue has been brought to your attention already but I thought I must post anyways.
Awesome game,
T