I'm looking at the description of the Hydra in Dungeon World (pg 248). It states that it has 16 HP and 2 Armor. So if it takes 16 points of damage it's dead. Seems straight forward.

However, It then says it can only be killed by a blow to the heart, and down the bottom though it talks about regeneration of body parts.

Not sure how to put these pieces together. Seems like unless they players actively state that they are attacking the heart, that the damage doesn't really matter.