@nitsua60 Like @Anaphory says, they still need HP.
The 16hp dragon was nigh invulnerable to the PCs, but still has those HP. Those HP are just not terribly accessible with just a party's hand tools. If they manage to drop a giant-sized cauldron off the top shelf onto the head of a giant snoozing in their giant kitchen, though, then you'll need some HP to deplete.