I normally say 1 HP but slightly higher Attack Damage, Armor Class, Typal Damage Reductions, and Save Proficiencies than normal for the CR range (though perhaps include one of those AC or Save Proficiency defence windows as lower, or introduce a specific set of damage types that override this system, a la the Pokémon Shedinja's 1 HP but only damaged by types its weak to – Fire, Rock, Flying, Ghost, or Dark types).
I'd just also caution that if not setting a weakness override, just don't set the AC/Save Profs/Damage Reduction Threshholds so high as to break 5e's bounded accuracy system.
I like to think of this as with a Musou/Warriors game, where we can cut through swarthes of enemies but only if we're properly leveled, using the right elemental type advantage (in games like Hyrule Warriors at least), and there are still base defender/captains/elites/hero unit warriors/giant bosses that do NOT go down in one hit.