okay i just wrote a huge essay about the neccessity of hit and run tactics on some mobs + good and bad examples of when to have and not to have regen and then i noticed that its not leading anywhere.
im against health reg for normal monsters but what you can do is the following: every monster that didnt take damage for 30 seconds (including bosses) is restored to full health, anything else would be, well, a bother.
but that really only goes for normal monsters. special or quest monsters should just stay the way they are right now, like the temple guardians in chapter 2.