My absolute favorite solution to the hunts that I would lol so hard over:

Trance mode/Panic mode for the mob. Like a fight or flight response most animals have in nature.
If over, say, 48 people are on the mob's aggro table, it gets a massive attack buff (2-3x damage) and near invulnerability until the number of people on the aggro list falls below the threshold or a timer (30 seconds or so) runs out on it.
When the timer runs out on it, and people keep pouring onto it, it simply flees and "vanishes", going to a different random area with the same non-recoverable HP pool.

Animals act like this when cornered. Why wouldn't apex-predators and masters of survival and combat?