I think I've seen it been called Finishing Blow, Killing blow and maybe Final Blow. Honestly, there are way too many names for the same thing but it always means the same "smack 'em while they are down" attack.
And yes, every single human opponent eats it, regardless of what buff states they are in, except when they are in the process of activating Living Weapon or some other similar buff up state. I've yet to find a human opponent who doesn't go down with it and like I mentioned previously, them getting knocked down will always also remove that buffed state as well.
There may be some obscure Hyper-Armor moments I've yet to test the attack on, but for the most part, it seems fail-proof barring those previously mentioned Living Weapon activation sequences.