The foe is nearly impossible to kill. Certainly, raw damage is not enough. No matter how many hit points of damage the creature absorbs, it is never properly slain. An undead creature, which cannot accumulate hit points, goes into a torpor when its stamina is reduced to zero, but is not actually destroyed. Eventually, so long as the creature is able to heal or repair itself in any way, it will get back up, and then probably seek vengeance upon those who harmed it.

However, the foe is only nearly impossible to kill. You must define a condition that will result in the creature's permanent death. This can be as simple (“The creature must be lit on fire”) or as complex (“The creature must be beheaded, its mouth stuffed with cloves of garlic, its body doused in holy water, and then both parts buried at a crossroads”) as you desire.