This sounds a little crazy, but I don't think we should all attack Yggrisalis straight away. If we do all attack straight away they (Cheesy111/Onizuka/UTAlan, assuming they are still Yggrisalis, which is practically a given) have some nasty responses up their sleeve such as:
On the other hand, we can't wait until we've all got a whole lot of Divine Actions to crush them with thanks to other Divine Actions such as:
Which will only get stronger as the game goes on. What we need to do is stagger our attacks either through a heap of cooperation, or for us to randomly Fight and use Divine Actions against Yggrisalis now for just about every turn until it dies. Co-operation through divine actions and healing other players would also be a really, really good idea (quite a few Divine Actions can be used for healing or for positive effects on other players).
But if we don't cooperate, we're pretty much doomed to lose.