Sorry, I suppose I didn't explain the idea well enough.
The monsters max hp will start out super high (if a lot of people have been hunting it recently).
Then if it's not killed anytime soon, the max hp starts to decrease to the current level of hp, where you can solo it in under a minute.

If not many people are hunting it for a while, its hp will start out low right away from there on, until more people start hunting it again.