Boss Health Formula
The boss of an area always has a base amount of health that is similar to a monster in the same area which is of the same type and level as the boss is, with an additional 5 health added for each level the player is.
Example for Creepy Undead Forest:
The Undead Champion is a level 12 undead skeleton so his base health is that of the Skeleton which is also level 12, an undead and obviously a skeleton, too. So the base health of the boss is 90 HP cause the skeletons health is also 90 HP. So for a level 8 player the boss would have 130 HP (+5 HP for each level), for a level 20 player 190 HP.
boss health = base health of similar monster in the area + (5x player level)