Scale hunt monsters up with local population.
Square, you repeatedly run into this problem where important monsters don't have enough health to survive large numbers of adventurers. It happens every time you implement something like this. You know it's going to continue happening every time, I don't know why you haven't learned yet.
You've put in this half-assed solution of making fates harder every time they're cleared and easier every time they're failed. Ditch that, put in place a system that detects how many players are in combat range and scale difficulty up dynamically with the number of players in combat with it. Crib notes from Rift. Look at how Guild Wars 2 handles its public events.
I got lucky with a high-class Hunt monster in Zahar'ak, it literally spawned on top of me. I got my ass kicked before anyone showed up, and in the minute and a half it took me to ride back from the nearby town, it was dead. I wouldn't be so agitated if this wasn't one of the big badasses that takes days to respawn.
Not cool, guys.