Enemies using AoE skills usually display a red circle indicating their attack. For Wyverns this does not occur. Why? We are used to red circles, orange circles, anything indicating a danger zone.
Yet for Wyverns you make them invisible which adds an arbitrary level of difficulty that shouldn’t be there. It just makes it really bothersome to fight them.
Well, more particularly, the Bloodstone-Crazed Wyvern. That one drops fire fields that deal massive damage in seconds, and there is no warning that the attack is imminent or where the fire will drop. This makes this boss impossible to defeat with a small group, you’d need an army and enough time on the timer.
If for some reason you have issues coding red circles for wyverns then at least increase the bloodstone wyvern boss event timer to 25 minutes so people can gather enough help to kill it in time.