Level 30 Harming or Healing potion kills any entities that are mobs regardless of health or effects, and kills players in Adventure/Survival mode, or Creative mode also regardless of health or effects.

Applying the effect with commands does not have the same effects, only the potion does.

Works with either harming or healing (or both), and works on boss mobs despite them being immune to effects.

Only works with amplifier 29/potion level 30, and rapidly throwing the potion can kill undead mobs. 

If it kills a player, there is no death message in chat, are their items are either deleted or kept in the inventory.

Killing undead mobs with the potion counts as a player kill, but it does not count for non-undead mobs.

Duration of effects does not matter. Command:

/give @p splash_potion{CustomPotionEffects:[

,{Id:7,Amplifier:29,Duration:1}]}