Pokémon Sun and Moon - How to use Pyukumuku

INNARDS OUT: When a Pokémon with this Ability faints, it deals damage to its opponent equal to the amount of HP it had left before last being hit.

So of course a mon with bad HP but high defenses is the perfect one for it!

I'm sure Gamefreak didn't want Wobbufet 2.0: The Landmine